The main reason is the particle し. As you would know, し is used to list clauses, and the topic (mostly subject) of each clause is often different from each other. In that sentence, the subject of 厳しい and 書かなきゃいけない are 先生 and 生徒, resectively. The speaker is enumerating the reasons of each topic (as for the teacher, he/she is stern, as for students, they have to write long reports, and so on), so it consequently has a nuance of contrasting. That's why は is preferred.
Incidentally, が is not wrong there. If the reason is only one or only about one subject, and the particle から is used to show the reason, が is most common.