ADA protects a student who cannot handle toileting. There is no way a state law could be written to prohibit attendance at a public school by a child (or a teenager) who is not toilet trained. The teacher might not (in a mainstream environment) be allowed to do it, so school would have to provide an aide.
It gets more complicated with private schools and/or preschools, but if the law mandates a child be in school the school has to handle toileting, and LRE means herding the child into a specialized program is not appropriate solution. Since Bear is 5 I am not sure any of this will help, but charters are bound by the same rules as public schools.
