American gunsmithing institute?
The Colorado School of Trades in Lakewood, CO (Denver area) has a decent reputation.
On-line schools always come with the danger of not being taken seriously:
How do you demonstrate your ability to operate a lathe or a mill, fit and bed a stock, mount & shim a scope, tune a trigger, straighten a barrel, and torque fasteners to the proper specifications on-line?
How do you learn hands-on from a master in a real shop on-line?
You can learn to be a lawyer or psychic on line, but physicians and gunsmiths have to learn by doing in the presence of experienced professionals.
Be well.