The question of what to ask for in a budget amount can vary by your
geographical region also.
I would find 2 similar high schools in your area and contact their art
teachers and ask them for an average year's budget amount. Most Art teachers
are happy to help someone get started.
At least that way you have some concrete data to go back to that principal
and make your case. Administrators are data driven.
Make sure to get data on how many students you will be teaching, vs how many
the comparison schools have.
Also the type of classses they teach (technology, clay, painting, weaving
Also include mounting supplies if they expect you to mount student work for
a district show.