Nothing ventured, nothing gained 🙂
I don't see the harm in you pursuing him and seeing where things take you; it's the 21st Century now, and unless you're of a certain religion(s), no one really cares who dates who anymore. Also, you're at an age when you still might be trying to work out who / what you are. Talking of which... rather than being full-on lesbian, do you think there's a possibility / chance you might be Bi-curious (but with a strong bias towards women)? Apparently even some gay guys will visit women prostitutes to confirm their sexuality LMFAO. Not sure if girls do this as well, but you sometimes hear guys (jokingly) saying, "I'd go 'gay' for XYZ"
Nothing wrong with telling him you find him attractive, and you'd like something to happen between the two of you; after all, having a crush isn't exactly a marriage proposal is it?!? Personally, I would say that you'd need to be dating someone for at least 2-3 months to know if it's going anywhere. You can be upfront with him about your history / sexuality and see if he's willing to date you in spite of that, or not say anything and make your excuses if you find that you do exclusively like girls after all.
I've always said that if people are more open minded, and are willing to date others who aren't their usual "type", they may find that they're compatible with a lot more people than they thought. They may also learn more about themselves, as that person may bring out different aspects of their personality / character. However, I've said this in the context of heterosexual relationships / couplings; still, I guess the logic still applies in your case?