You know what? You are PAYING to take the OW class. Get the BP/W, get it adjusted to fit and just show up with it. You are the customer, end of discussion. If the instructor is incapable of teaching you to dive just because you have a BP/W, get another instructor. There are literally THOUSANDS of instructors. Far more than the sport could ever use. Someone will be qualified, capable and willing.
In fact, consider taking the class privately or with just your dad. Sure, it will cost a little more but the training will be more individualized. Classes for juniors are frequently one-on-one and the cost is $400 versus about $170. It's more but, in SCUBA dollars, insignificant.
A 'boat buck' describes the monetary unit when messing about with boats. It's actual dollar value is $1000. So, if you spent 5 'boat bucks' on maintenance this year, it was really $5000. It's the same kind of thing with SCUBA dollars but it probably converts in units of $100. So, a private course is 4 SCUBA dollars versus 2 SCUBA dollars for the conventional approach. We use these virtual units to keep from crying about the real cost!
Consider doing the checkout dives from a boat. Sure, most courses probably include beach entries and boat dives may cost more but who cares? You are the customer! The instructor needs to make it work for you. Even if you have to charter a 6 pack boat for a couple of days, who cares? Organize your training to fit your needs.
You do not have to apologize for anything when you are the customer! Things are what they are. Go for it!
Richard