So, it seems like your partner really wants to amp things up, but school is weighing heavy on your mind right now and sex is the last thing you feel up to doing right now. How do you say no without offending the person?
Honesty! Tell the person what's going on- chances are s/he will understand and appreciate the truth more than anything else.
Having sex should be fun for all parties involved and each person should have equal say in what's happening and be of sound mind to participate. If you aren't into it for whatever reason (i.e. family issues, friend drama, midterm mania) let the person know that now is a crappy time for you. Or if you can't think of something right away, try excusing yourself to figure out how you want to say it. And remember, the person you are with should be accepting of your not wanting to have sex. That's part of being in a relationship -- respect.