A teacher is now allowed to create up to 100 students in a student group. This assumes that the teacher is a plus member. If the teacher is not yet a plus member, the limit is 50 students per student group.
Remember, you can create 20 groups as a teacher, no matter if you are a plus member or not. If you get a new class of students, you should create a new student group, instead of adding students to the existing group.
Read more in the Help section for creating a group.
Let us know if you have anything else you miss or would like us to look into. We are always looking forward to your suggestions for improvement.