I get random phone calls all of the time about the same thing. If I get someone, I just let them know I'm already enrolled in school and they don't call me anymore.
When you tell them to take you off their list, they won't. So just tell them you're in school already. You may get other phone calls from different numbers for the same thing, but I just tell them all the same excuse. I'm in school. They usually hold off for about a few days on calling.