Tips for saying "no" :
1. Practice in your mind the scenario.
2. Just say "No" and nothing else.
3. If they keep pushing, have ready, reasonable excuses.
4. Don't feel guilty.
5. Get call display and don't answer the phone lol.
6. Hide behind the furniture when someone knocks.

Honestly though it's a good thing to learn how to say no. Take it easy with the old noggin - you need it for good grades.