Legal Question in Business Law in California

Re-selling bus tickets is inherently a legal business if and when conducted according to (1) the rules and laws that apply to sales businesses in general; and (2) any special rules and laws that govern this particular kind of business, which may be regulated by a Federal or state agency with authority over transportation of passengers, such as the Surface Transportation Board or the California Public Utilities Commission.

It isn't clear from your question exactly what your relationship with the bus operator will be. If the bus operation is also a new business idea thought up by some students, be sure that they understand that the transportation of passengers for hire is a regulated business and will perhaps need to apply for and receive a "certificate of public convenience and necessity" or similar operating authority before it can function. It will also need insurance.

A reseller of tickets should, ideally, have a working relationship with the issuer of the tickets. Reselling transportation tickets isn't as simple as reselling concert tickets. The bus service may want to control the down-stream redistribution of its tickets, and may have that right.

So, the business idea is legal, but subject to a variety of possible restrictions, public and private. Like any business, it would need to keep books and records and file tax returns, and hold the necessary licenses and permits. The decision to form a corporation or LLC is optional, but you should understand why, or why not, and organize accordingly.