I use ubuntu 14.04 and made 120 users. 115 users are in "student" and 5 users are in "assistant".
If I delete groups, then are users in those groups deleted too?
You may not remove the primary group of any existing user. You must remove the user before you remove the group. (see gropudel documentation)
No, as long as the user has a primary group which is different to "student" or "assistant" then they can be deleted easily however if these groups are the users primary groups then you will not be able to delete them without giving the users a new primary group.