What subjects do you need to take when in high school to become an interior designer?

This answer is completely dependent upon the school - each has differing requirements for degree, so review each carefully. Schools with an emphasis in art and design may not suit you as well as schools which stress family studies and communications. Make sure that your education is as broad as possible, and take every opportunity to intern in different fields; interior design as a profession can lead you into many areas (this writer has worked in corporate, retail, residential and hospitality design in about 15 years). Technology continues to take us into bold new directions, and adaptability will always be crucial for success. One important consideration is accreditation in your field, as some schools which offer design degrees are not accredited by any Interior Design professional organizations - and, after all, it is these professionals who are going to be looking at your resume after graduation. Personally, I would favor a school which offers classes in construction and engineering, as designers in the real world need to be conversant with How Things Work. Remember, good design must be able to be built - and professionals in the building trades will respect you if you show some understanding of their work. Good luck, and welcome to the trade!