Foster children are not a protected class under the law.
Perhaps foster kids could or even should be a protected class, however unlike most protected classes that have faced historical systematic discrimination codified in law, the general hardships of foster children are not based in unjust laws.
I have worked in Dependency law (ie with children that have been abused, abandoned and neglected) which deals a lot with foster kids.
I favor programs that provide funding for foster kids like this and provide assistance when they “age out” of care, but it is a broad brushstroke and doesn’t take into consideration individual situations as you suggest. In other words foster children are not all alike nor are their situations. Some live in group homes and they are just a number or a check for foster parents, some live in loving and supportive homes, even sometimes in the homes of relatives when parental rights were lost but they are still considered foster children. Some become foster kids at 17 and others are born into it. There is everything in between.
It is about the equivalent in terms of diversity of situations as being a minority/protected class that has historically been discriminated against.
And even if they have an almost adopted type of situation with their final foster parents, those parents still shouldn't have to pay all the college costs. They may not have even had the children placed with them long enough to save for that.
I mentioned in another reply, in Florida we have programs that offer financial assistance to kids that age out and continue their education. I support the programs, but there are things that should be acknowledged like it resulting in foster kids not getting adopted that otherwise would as financial strategy, or kids going into foster care right before aging out to qualify for the program.