Well, as long as you like Blondie for her looks AND intelligence, that's a good sign. If you only liked her for her looks, then I'd suggest you rethink this situation.
You said that last week was the last day of school? In my opinion, take advantage of the summer break and try to get to know both better. Perhaps organize a group thing to a water park or miniature golf or something and include one of the girls, then arrange another activity and invite the other. I'm quite a bit older than high school, but some of my best times were during the summer between school terms. It was more relaxed and you didn't have all the cliques to worry about.
Some of the guys I dated started out as friends just hanging out. The big thing is to be yourself. If you're not, she'll see through it and won't be comfortable around you... believe me, we CAN tell!
On a final note... what's the rush to "date" somebody. Perhaps I don't know what your definition of "date" is, but I see it as one guy and one girl seeing each other exclusively. Personally, I think you need to go out with someone more than a couple of times before deciding if you're dating. When I was in junior high we "went with each other" which boiled down to being "boyfriend/girlfriend" based solely on superficial things, and the most you went out was to hang out at school or a school function, pass notes to each other and maybe hold hands or sneak a kiss. If you're really interested in dating and actually going out with someone, then get to know her and see where it leads.
Oh, as far as how you can tell if a girl likes you... from what I can gather about your age group (or guess at) I'd say if she shows interest in you and takes the time to talk to you or acknowledge you then that's a good sign. If she acts like you don't exist or doesn't seem to know if you've come into or left a room, then she's probably not interested. That's when you try the group activity thing and invite her along (and don't invite any rivals

)
Good luck!

CG