Adolescents experience relatively more stress than other populations as they are facing rapid physical changes and adapting to complex social environments. However, access for this population to professional service providers is limited. Therefore, there is an increasing need for access to mental health services and new mental health care resources tailored to adolescents. The aim of this study was to evaluate the functionality and effectiveness of a school digital mental health clinic (DMHC) created by a Chinese psychiatric hospital and provided to secondary school students for a trial. The trial period of the DMHC was from January to July 2021 at three secondary schools in Taizhou City, China. Under a collaborative agreement between the local educational bureau and provider, use of the DMHC was free to all students, teachers, and staff of the schools. The functionality of the DMHC was compared with existing digital health interventions introduced in the literature and its effectiveness was quantitatively analyzed in terms of the volume of received counseling calls, number of calls per 100 students, length and time of calls, and reasons for the calls. The mini course video views were analyzed by topics and viewing time. The design functions of the DMHC are well aligned with required factors defined in the literature. The first advantage of this DMHC is its high accessibility to students in the three schools. All functions of the DMHC are free to use by students, thereby eliminating the economic barriers to seeking and receiving care. Students can receive virtual counseling during or after regular working hours. Acceptability of the DHMC was further ensured by the full support from a national top-tier mental health facility. Any audio or video call from a student user would connect them to a live, qualified professional (ie, a psychiatrist or psychologist). Options are provided to view and listen to resources for stress relief or tips to help address mental health needs. The major reasons for the counseling calls included difficulties in learning, interpersonal relationships, and emotional distress. The three topics with the highest level of interest for the mini course videos were emotional assistance, personal growth, and family member relationships. The DMHC served as an effective tool for crisis prevention and intervention during nonworking hours as most of the live calls and mini video viewing occurred after school or over the weekend. Furthermore, the DMHC helped three students at high risk for suicide and self-injury through live-call intervention. The DMHC is an effective complementary solution to improve access to professional mental health care facilities, especially during nonworking hours, thereby helping adolescents meet their mental health needs. Extension of the DMHC into more schools and other settings is recommended.