Those kids that are into online gambling. they are hard to control because how secretive they are whenever they are asked. So, for everyone that gets the idea of how these bets from the example of op coming to the kids and teens that are gambling secretly. I agree that parents need to step up and always remind them with all their might not to engage into such activities.
Unfortunately, despite being strict with their parents, there was a time when they couldn't monitor their kids.
But, as mentioned by OP, it was already in high grade, which means that he knows what he is doing. He is old enough, and his parents never expect this to happen, as they believe he is doing right. And many scenarios like this, not just in gambling but also in other things, where parents confidently believe that their kids are following what they say. It means that we can't completely control everything, even if we talk about it many times.