Summertime, I'll wear capris, and since I've got a decent hill on my way to school, I'll probably change my shirt.

Wintertime, if I can't change I'll wear long johns under jeans down to about -10 C. However, I prefer cycling tights - as DIMcyclist said, jeans wear out pretty quickly. On top I'll wear a long sleeve merino shirt + a few layers of jackets.

I would check Goodwill/your local thrift shop for clothes. Sometimes you can luck out and get some technical clothing, but if not, it's a good cheap source of jeans that you are probably going to destroy.

Finally: get full fenders. Not only will they protect your clothes, but they'll also help to protect your bike.