I am not sure I have the answer to your question there and I'd bet that the union hall doesn't either. I'd recommend that you ask the company for a copy of the contract and see what it says. I would hope that there would be some language to address your situation in that contract but wouldn't be surprised if there was none, they do like to be one-sided in their dealings. I would be upfront and address this with whoever is offering the school to you, but that's just me. Do they have any drivers who are ready to retire soon? That could be one reason they might want to line you up for seniority.