There are schools that teach textile design which is about creating fabrics. You should look into that area of design if you want to design fabric ... not fashion design. We have lots' of threads about textiles and textile designs ... just search thread titles only and you'll find them. Here is one about schools for learning this:
Textile Design ... Schools
If you are interested in printing designs or logos on fabric ... like on T-shirts, hoodies and bags, etc., then graphic design would be of interest to you ... which you'd most likely find in an art school.
If you plan to actually make the pants, shirts, jackets, etc. ... .then fashion design is still necessary because you need to learn to make patterns, sew them, fit them, etc. Whether it's a high end gown or a pair of cargo pants .... the process of designing and creating them are the same ... just a different aesthetic.
And if you want to be sucessful in business, then you need to learn about financing, marketing, advertising, sourcing, markups and profit and loss.
So, if you can find a school that offers some instruction in all or most of these, I'd say that would be a good start. You'd get a very basic understanding about each, and then, could further your education later in the areas that you want to concentrate in.