Provided the buses are suitable for everyone who needs to use it, there is nothing wrong with 30-40 year old buses on school routes. It is obviously easier to determine what those requirements are if the provision is a private charter for a specific group of individuals, such as the pupils enrolled at a school, rather than a public service, and it is irrelevant whether the school in question is private or state.
What would be illegal is to refuse to enrol a new pupil simply because the school bus is unsuitable. The legally required "reasonable adjustment" would be to provide a suitable vehicle, either by replacing the school bus with a wheelchair-accessible one, or by making alternative arrangements for that pupil, such as a taxi.