You might also try /r/AskAnthropology. Good luck.

300? That seems a bit excessive, or else I am not understanding what your teacher expects. As far as the written form goes, it is written with a mix of standard and accented Latin alphabet characters - the same ones used in English. I'm not aware of any ideographic version where there would be a real library of symbols to be had, nor of any pre-European-contact written form.

Tlingit culture does produce totem poles, and they share the rich artistic style of the northwest coastal cultures, manifesting in blankets, masks, painting, and crafts, but to assign specific meaning to 300 different symbols as part of a school project just seems a bit unreasonable.

You may be able to find some information on Native-Languages, which does have some parts specifically meant for students and educators. The only others sources to check I can think of (which mostly might just be places to find knowledgeable people to contact) would be the Central Council of the Tlingit and Haida Indian Tribes of Alaska or looking up Tlingit artists that could be contacted, or who have a significant online presence. The National Museum of the American Indian (Smithsonian, in DC) has a section by the Tlingit, but I honestly don't believe there are even 300 objects on display there, much less 300 interpreted symbols.