Move on as soon as you're comfortable, never before. No need to be in any rush, take your time with things like that in life and avoid rash decisions. If you feel like you want to be in a relationship then it's alright to go ahead and move on whenever you are ready.
As far as your situation specifically, I can't offer much advice without knowing age and what you're are looking for in your life right now. As an example, I'm 21 working part time and in my last year of college. I'm single and glad that I'm that way, because I don't think I personally have the time to invest into a relationship. But that is just me. I'd go with the girl that I know the most about and felt the most comfortable around. If the girl in English seems to be interested in you try asking her out for coffee or try to start up a conversation. If the girl in Math doesn't seem to be interested, why is that? Do you two just not talk? Does she seem really preoccupied?
What matters most is your comfort with where you are in life and what you believe will make you the happiest. Take risks, make gambles, but always make sure that when you are, you're doing those things to make you happy and you're doing them with a solid conscience and a clear, calm head.