I haven't read through the thread so I might be repeating, but... YES school activities are expensive! But not NEARLY as expensive as comparable activities outside of school. in my experience. For instance, if you wanted to take advanced language classes OUTSIDE of school.... you'd be buying those two books PLUS ALL OTHER CLASS MATERIALS and paying for the class itself and taking time after work, in the evenings, to do it.
Band stuff... I don't think there are a lot of parallels to "marchng band" outside of school (maybe I'm wrong?

) but I bet a lot of the equipment required is school-owned or obtained through the school at a discounted rate. Ditto for school sports. WAAAAY cheaper than private sports leagues.
Bottom line... Childhood is a time to experience all that you can, try out everything that interests you, and darn it, it sure is expensive.
"Acceptable" clothes... we all want these. Looking inconspicuously normal is a goal for many teens; expressing their own style is a goal for others. The budget for school clothes and shoes needs to be realistic within your budget but also realistic in regard to what basic accoutrements are going to cost. It's great for kids to supplement their wardrobe with earned money but within your ability to do so,
you the parent have to provide acceptable basic wardrobe. It's a lot harder when they are 14 and not 2, because they have a lot more opinion about what constitutes an acceptable wardrobe.