.025g is half a point. Do about... Just enough to thinly coat about 1/3 of a regular table spoon, I guess. And the first rush is the best, yes, but remember that you can always do more but not less. That first shot will tell you where you sit. And quality varies a lot, so if you happened to get really good coke you don't want to be having a heart attack by yourself.
Can you be at least outside or somewhat in public? I know that doesn't make sense... I just worry about you being alone. Maybe arrange for a friend to come over at a certain time right after you do it... and if you are fine, you can tell them not to come or something?
I'll be honest, if you get any sort of substantial amount you will probably do all of it. So only have access to as much as you want to allow yourself to do.
If you do a shot or two today, you should be able to go to school. I don't know about sleep. Like I said if you can have some opiate or benzo around to help you sleep that would be good.