The transition from the 9th grade to the 10th grade0 in Taiwan could be one of the most stressful times for many Taiwanese adolescents. This involves leaving the junior high school that they were familiar with and entering a new school environment (i.e., a senior high school, a vocational school, or a junior college). Adolescents face various academic and social challenges in the new environment (e.g., learning new curriculums, getting to know new teachers, and establishing new friendship network). Findings of previous studies have revealed that school-related concerns have troubled many Taiwanese adolescents (Chen, 2006; Liu, 2004; Ou, 2012; Tung, 2018; Wang, 2007). The ability to adjust in the new school environment increases the chance of successfully entering the emerging adulthood. Furthermore, school maladjustment increases the risk of dropout, mental health problems, and/or misbehaviors (King, 2003; Henry et al., 2009; Maynard et al., 2015). Studies have revealed that parental and peer attachment (Al-Yagon et al., 2016; Laible et al., 2000; Lim & Lee, 2017), active coping (Lu, 2014; Merlo & Lakey, 2007), and the level of mental health in the past (Bond et al., 2007; Duchesne & Ratelle, 2014) significantly contribute to adolescent school adjustment. However, most previous studies have adopted a cross-sectional design and have overlooked variables when considering the effect of time on adjustment. This study adopted a longitudinal panel design approach. The author intended to examine the predicting effects of parental attachment and depression at 9th grade as well as the effects of peer attachment and active coping strategies at 10th grade on adolescents’ school adjustment. Specifically, this study aimed to (1) examine the relationship between parental attachment and peer attachment and (2) explore predicting factors for adolescent school adjustment during their transition from a junior high school to a senior high school, a vocational school, or a junior college. Of the sample of 229 adolescents, 161 (70.3%) were girls and 68 (29.7%) were boys. Except 11 participants who did not report their age, the mean age of the rest of the participants was 16.38 years (range = 15.75–17.75 years, SD = 0.33). Furthermore, 104 (45.4%) participants attended a senior high school, 82 (35.8%) were from a vocational school, and 42 (18.3%) were from a junior college, a comprehensive high school, or other types of schools. The Relationship Structure Questionnaire-Chinese (RSQ-C; Wang & Neville, 2006), Coping Strategies Scale-Chinese (CSS-C; Ji, 2002), Adolescents’ Behavioral Disturbance Scale-Chinese (ABDS-C; Lin, 2002), Adolescent Depression Scale (ADS; John Tung Foundation, n.d.), and a demographic questionnaire were administered. The Cronbach’s alpha coefficients of these scales were above .73. Participants completed the surveys twice. Participants’ levels of attachment anxiety and attachment avoidance toward their mother and toward their father as well as levels of depression were assessed when they were in the 9th grade. Thereafter, participants’ levels of attachment anxiety and attachment avoidance toward their best friend and levels of using active coping strategies were assessed one year later when they entered the 10th grade. These eight variables served as predictors, and participants’ levels of school life disturbance in the 10th grade served as the criterion variable. Furthermore, the Pearson product-moment correlation and the stepwise multiple regression analysis were performed to test the research hypotheses. The study findings indicated that adolescents’ levels of attachment anxiety toward mother and father (Time 1) significantly related to attachment anxiety toward the best friends (Time 2) (r = .39, .32, p < .01) and attachment avoidance toward the best friends (Time 2) (r = .15, .14, p < .05). Moreover, higher levels of attachment avoidance and attachment anxiety toward parents were associated with higher levels of depression (Time 1) (r = .33, .17, .35, .20, p < .01). In addition, depression (Time 1), attachment anxiety toward the best friend (Time 2), and use of active coping strategies (Time 2) significantly predicted adolescents’ school life disturbance (Time 2) (β = .21, .22, -.34, p < .001) (F = 29.86, p < .001). The cumulative explained variance of these three variables on adolescents’ school life disturbance was 29%. Surprisingly, two dimensions of attachment toward mother and father (Time 1) were not significant predictors of school life disturbance (Time 2). Furthermore, attachment avoidance toward both parents (Time 1) was not related to two dimensions of attachment toward the best friends (Time 2). Consistent with the literature, this study supports that prior mental health status significantly predicts adolescent school adjustment. It is hypothesized that adolescents with high levels of depressive mood in 9th grade may show deficits in executive function and attention. They may also become socially withdrawing. These cognitive and interpersonal difficulties hinder their abilities of handling academic challenges and seeking social support in the new school environment, thereby leading to school life disturbance. Additionally, the findings support the differential effects of two dimensions of peer attachment on adolescent school adjustment. Although attachment anxiety toward the best friend significantly predicts school life disturbance, attachment avoidance toward the best friend does not. Earlier studies have mostly examined the effects of peer attachment on adolescents’ negative emotion. Studies exploring the influences of peer attachment on adolescents’ school adjustment have been inadequate. Accordingly, this study added evidence to the current literature concerning impacts of peer attachment on adolescent school adjustment. This study did not find attachment avoidance toward the best friend to significantly predict school adjustment. This may be because Taiwanese adolescents’ interpersonal interactions with others are guided by cultural rules and based on types of interpersonal relationship. Therefore, although adolescents may request assistance from friends, when encountering real difficulties, they still seek help from parents and siblings instead of friends as kinship is stronger than friendship. The author assumed that restraining from requesting support from friends reduces the influence of attachment avoidance toward the best friend on adolescents’ school life disturbance. Finally, the most significant predictor is active coping strategy. This result echoes the findings of previous studies regarding the influence of coping on individual adjustment. When adolescents actively adopt a problem-focused coping strategy to address issues arising during the process of adjusting to the new school environment, their levels of school life disturbance are expected to be low. Unexpectedly, two dimensions of parental attachment in the 9th grade did not significantly predict adolescents’ school maladjustment in the 10th grade. As adolescents get older, peer attachment plays a more important role in individuals’ adjustment than parental attachment. Furthermore, peer attachment exerts higher levels of influences than parental attachment. The reason may be that parental attachment was measured in the 9th grade and was significantly related to depressive mood, attachment anxiety toward the best friends, and active coping. These three variables significantly predicted adolescent school life disturbance. In stepwise multiple regression, the variable was excluded from the final prediction model for not adding more explained variance to the criterion variable. Further studies are required to provide evidences on these postulations. The study findings highlight the influence of prior depressive mood, attachment anxiety toward the best friend, and active coping on adolescent school adjustment. Based on these findings, the researcher recommends intervention programs for teaching active coping strategy and for facilitating peer relationships for 10th graders.