Being an art student, I too had a huge amount of stuff to take back with me. The thing to do is to send as much of your clothing, books, etc. as you can via the regular mail. If you can send things for which you can wait a few weeks, extra clothing, or say winter stuff, send it by surface mail, "via barco" and it won't cost much. That way you can reduce your baggage count. If the flight is not full, ask if you can strap your guitar (in its case of course) into the seat next to you, like a regular passenger. The thing they do not want is to have loose potential projectiles lying about the cabin (hence the "put it under the seat in front of you or in the closet").