S.P.
Hi J.,
I know this is not much comfort to you, but the only advice I can give you is what I had to learn the hard way. You have to get the paint out while it is still wet on the clothing. Since this is happening in her art class, I would kindly tell the teacher that you have had to replace several uniforms because this paint won't come out. Maybe he or she would try to ensure that the paint either gets washed off while it's still wet, or make sure her uniform is sufficiently covered with an oversized t-shirt or bib. You could even bring one in and request that she wear it during painting. I would imagine that the art teacher would be very understanding.
I have never found a product that got the paint off after it dried. I would also love to hear of a product that works.