As for the general question for "When to approach girls?", I agree with Light. I say whenever the opportunity strikes, and this can be any time. I'd say this is a very important lesson to learn for later in life.
But, as for high school, I always approached between classes. I didn't know a single thing about seduction then, but I found this the easiest time for me personally. I'd catch her at the locker or talk with her while leaving class and ask her. Why? Because then if she rejected me, no one else would be around to hear it haha. And yes, I got rejected, but I also got girls that said yes. A man without failure has never had success.
As for time of day, if I only had her for my 1st morning class, then I asked her on the way to my 2nd class (even going in the opposite direction of my actual class to ask). It just depended on when I could see her.
In high school, everyone is kind of learning, so I don't think it really matters much. On Valentine's day, I even bought a girl a box of chocolates and got a date. Yes, I did a lot of stupid stuff growing up. However, I'd avoid asking in a note:
"Will you date me? Please circle one of the following: yes no maybe"
Although, I actually did this in college as a joke (she already knew I was a goofball from hanging out). I actually got a first date, but no second (for obvious reasons now). I honestly didn't think I'd even get a first date out of it; I was semi-interested and thought it would be funny.
In high school, the main thing is to just do it. If you think about it for too long, you eventually psych yourself out. I didn't ask out a lot of girls that I should have. And, do not be that guy that asks out like 10 girls at once in one day since gossip spreads like wildfire in high school. Let's say you ask out a girl and she says no, but now you want to ask out her friend. Her friend knows that you asked out her friend before her. You're going to have to wait.