TeenHealthFX understands that you are really in a tough spot. No one likes to be made fun of or treated badly; and sticking up for oneself is important - but you are right, a physical fight carries many consequences. FX thinks it is very mature of you to be worried about these consequences. Getting kicked out of school is very serious, not to mention the physical danger you and your opponent will be putting yourselves in. However, FX also knows that walking away from a fight is easier said then done. It takes a brave person (not a coward) to walk away from a physical confrontation, when getting involved is a whole lot easier.
So, if possible, TeenHealthFX recommends continuing your strategy of walking away and avoiding future confrontations with these girls or attempting to work out the problems with the help of a teacher or school counselor. If you think what they are doing is getting worse, like bullying you, see our answer to Bullied And Afraid It Will Continue.