You can go back to school at any time. According to the relevant regulations of China's Ministry of Education on students' suspension from school, as long as they go through the normal suspension procedures, they can return to school after they come back from school. As for whether you can re-enter senior one after you come back from school, you need to discuss with the school and make a suitable plan according to your study. "Regulations on the Management of Students in Ordinary Colleges and Universities" Article 25 Students may complete their studies by stages, and unless otherwise stipulated, they shall complete their studies within the longest study period stipulated by the school (including suspension and retention of school status). Students who apply for suspension from school or who the school thinks should be suspended from school may be suspended from school with the consent of the school. The number and duration of suspension from school shall be stipulated by the school. "Measures for the Administration of Student Status of Primary and Secondary School Students" Article 19 If a student drops out of school, his/her parents or other guardians shall submit a written application, and with the consent of the school, he/she shall report it to the competent department of student status for registration through the electronic student status system. When resuming school, the school should handle the relevant formalities in time. During the period of suspension, the school should retain its student status.