Kudos for making good choices for yourself and not looking to others to make decisions for you. That shows that you have confidence in yourself and that you respect yourself.
I am not sure what you mean by tights because pantyhose comes in a variety of thicknesses. So if you are going from boby sox to full leg coverings, are you necessarily speaking about heavy dark tights or just full coverage? Aren’t there different thicknesses you can try for summer and winter?
As far as skirt lengths are concerned, you can ask your school if there are any “extra” uniform skirts available and/or try a gemach or even ebay to get a gently worn longer skirt in good condition. I am sure that if you looked in the back of your local newspaper you might find a gemach or ads advertising “give aways”. Don’t worry about what others think or being teased about being a “frummie” if anyone says that to you say “thank you” and be proud to wear that label. It is so much better than Donna Karan or Calvin Klein.