Full Time Student rating

My daughter is turning 21 next year. Just got a letter that she will now be rated as an adult dependent. The more I argue that she is a full time student and her status of dependency is no different to that of an 17 year school going child the more they argue it makes no difference. My contention of this dispute is based on dependency. Since any adult is rated the same as my daughter I cannot see how change of potential health risk can necessitate such change. The pricing of a full time studying student should just be capped at age 26 with the same rating as that of a dependent child.