Is this a good work out routine?!

Is this a good work out routine?

After I walk home from summer school (about 3/4 a mile up hill)

I get on the treadmill, starting out by running 12 minutes straight, then briskly walking 15 minutes, then running 6 minutes, and finally finishing out with walking briskly again for 15 minutes. Total of 3 miles.

100 good sit ups

lift weights for my arms

A little jump roping (about 100 jumps)

50 squats

I usually bike about a half an hour (about a mile or 2)

sounds like you've got it all under control :)
just keep in mind that you wont see results unless you have good eating habits. and if you eat like you workout, you'll be in perfect shape in no time!!
good luck, i hope all goes well!!