I haven't put my foot in the online-game water yet, but I'll give it a shot...
I'm going to say that if you see her in person often (in History class), then that is when you should be gaming her the most. Online interaction can be helpful, but use it as only a tool to boost the game you already have with her, not a primary asset since you do have the in-person aspect going on for you. When talking to her on myspace, I'd keep it short, funny/witty, upbeat, and to the point. Keep most of the more serious conversation for in-person. She won't be able to get a feel for you personality/tonality over text, so just use it to set stages and then take it from there in person. It's good to use smiley/emoticons to try to better convey the tone of the responses.
When setting the date, which you said is becoming tricky due to schedule conflicts, take the initiative. Next time you talk to her, have a date in mind that works for you. Shoot her an myspace IM or message and be playful. Keep it fun. Then just say something like, "This Friday at 7pm I'm going to pick you up and we're going to go to _________ and _________." In my experience, girls love when you don't ask her on a date, but just do it. In fact, last summer I met a girl and called her a few days later. I told her pretty much what I told you to say. I told her I was going to pick her up that Friday and take her to a party my friend was having. Before even agreeing or disagreeing to go, she said she loved how I just called her and told her what we were doing without "asking" her out, and yes, she went. She was not being sarcastic when she said she loved it either. Most guys are too unsure of themselves and will hesitantly ask a girl on a date. If you do it with confidence and initiative, she will love it - it's "ballsy" so to speak. Example:
"My friend is having an awesome party Friday night and we're totally going to go and have a blast! I'll pick you up around 7pm!" If this is done in an enthusiastic manner, who wouldn't want to go to a party and have a great time. It sounds like a set plan too, so why not just go with it? Versus:
"If you're not busy on Friday, do you think you would maybe want to come to a party with me?" This can work too, but to me it shows too much uncertainty. Now she can just simply say "No", or if she finds something better to do she may blow this off since it's not really a set in stone plan.
If she's busy and can't make the date you set, don't get upset or discouraged. Tell her you are also busy other than that date and you'll have to get back to her about it then. If she suggests a date, then go for it.
As far as creating more attraction through online interaction, I would focus on just being creative, fun, and different. Don't give normal everyday responses to questions like "What's up?" Every guy and his brother responds with "Nothing" or "Just chillin'. How about you?". Also, think of the profile aspect of it. Don't allow your profile to portray you as a boring or cluttered (with a million banners and useless information on myspace) person. Too many people cram paragraphs and movie banner images across their profile. No one will know what to look at and it's not unique. Your profile pictures should convey you having fun and being social. Try to stray away from the typical mirror shots and solo pictures. Instead have yourself out and with other people... a picture with other girls doesn't hurt either. Remember, you are a fun and social person who everyone loves to be around.
Again, I'm not typically on online gamer, but those are some ideas that come to mind when reading your post. You will probably have better luck and advice by posting your question and similar ones on the Online Sarging section of the forums. Chances are there are members on this forum who frequent that section and have a better understanding of online game.
Hope I helped a little
CHAT