Express Checkout Billing Agreements and Reference Transactions would get this done for you. You'll need to get Billing Agreements enabled on your account, and then you could set it up so that your teachers create a Billing Agreement with your site when they sign up, or through their profile after the fact. Then you could use Express Checkout to have payments sent directly to the teacher account (equivalent of primary receiver in Adaptive Payments), and follow that up with a DoReferenceTransaction request that would pull your commission from the teacher's account using the Billing Agreement ID. You could do this in real-time as part of the checkout, or you could keep a running balance for what the teacher owes you and then use a CRON job on your server to run once per day and process all commissions at $100+, for example. That's the best solution you'll have immediately available. Otherwise, you'll need to wait on PayPal Marketplaces.
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