You first need to establish that she is in fact pregnant. She needs to take a pregnancy test.

Once she has done the test and its 100% just talk to them at night and say "well guys i screwed up. [that girl] is going to have my baby." you have to be there for her and cant dump her for that. child support isn't very expensive and if you broke up she can sue for child support.

do you want to keep the baby or not? this could influence how your conversation goes. you should have some basic plan for your future with this baby so that you can answer their questions and there will be A LOT of questions, like how are you going to afford it? are you staying in school? are you getting a job? what about your plans for the future? etc.
it is best to have these conversations face to face, but sometimes due to anxiety or a volitile household you may want to have a friend there or even write them a letter and give them some time to read it.
you could tell them that you crashed their car into an expensive art gallery and that you owe $850,000 in damages and then tell them the truth so in relation it wont seem so bad kidding.
if you do talk to them face to face, i think you should say something to the effect of "im so sorry, i know your going to be dissappointed but im really scared and i need you right now...."etc. or you could say something that kindof takes the edge off if you have a hard time spitting out "we going to have a baby", like "guess who has a bun in the oven? we do" or "i sure hope you remember how to change diapers, cause were in for a wild ride...", "you sure look great for a grandma?" etc. find a way that your most comfortable with and good luck.
ps. it always helps to throw some compliments in there like "you do such a good job as a parent, maybe you can help me learn" or something cheesy like that.