I'm going to help a girl with school work for a blowjob
I'm not really sure why you would ask her to do that?
She's pretty hot.
That's horrible of you!  It's fine to want to help people, but not at the price of them performing some unwanted sex act on you!  Ask them for a few bucks for your time or something.
We can do this with or without sex, I promise.
You should definitely do it without sex, as there's nothing right about trying to talk someone into having sex with you over help. Be a good person, and if she likes you then you can pursue sex as a natural thing instead of a coercion.