The logic doesn't quite match. Dial up uses the telephone line right. Ethernet, well ... thru your school which will use something dramatic whatever it is. For a dial up modem you need to be using a dial up service thru your isp. Also your modem is designed for 56kbps (approx 7KB?s max speed.) YOu can try it but I have my doubts. You should try connecting thru their ethernet if it's allowed. Either that or buy yourself a service. Be it dial up or broadband, etc.

You'll notice your comp has both an ethernet card & connection & a dial up connection at the back. Notice the difference?