What's a mutator ?
Seriously, you'll have to tell us concretely what your problem is, not just write your assignement like that. Remember that the ranch is
NotACodeMill. If there are many problems, break them into small problems.
1. Where do you add the signUp method ? What's its parameters ?
2. Increment the student's achievement points
3. Add the student to the team
4. Increase contest's total costs (value depends on the age of the student)