I'll add my thoughts. Continual discussion would be my path - this could be a little more of a softer "nudge" from time to time, but not all of the time. Add a touch of silent monitoring too. And find a way to prepare them to balance and time manage - because this will be an invaluable skill once they make it to the collegiate level.
I also have a college player, and I will loudly second that it is a lot of work to play baseball at the college level and get classes and homework in no matter how good the player. At the collegiate level, all players are usually very very good, and it takes talent and hard work to get through it all. There is little time for anything else - even sleep sometimes, and eating too. To be at this level, the player must want it and understand the huge amount of work involved. Now, I am sure that your kids all want it very badly, but it is human nature to need free time. So that is why I suggest during HS continually talking with your sons about how they feel about what they are doing. On the parental level, assure them that you are there to make it all happen for them - to do what you need to do as a parent - pay the travel fees, buy the equipment, and make sure they get everywhere they need to be, but the work ethic must come from them - not you.
Regarding baseball: TPM says time off in the summer - yes, I agree. As a younger player, take more time off. Your junior summer, the player will likely get little time off if he wants to be on the radar. Make sure he understands this level of commitment through your discussions. And it does not stop in HS, as during summers they will play in collegiate leagues, away from home, and living with host families. My son (a soph) said his summer last year was the best of his life so far. Not all get this great experience, and if they don't they must deal with it. At the HS level, the player needs to level it up each season, because he wants to be on the coaching radar to get to play in college. It can get intense with all of the camps, showcases, scout ball, and such, and they must be ready for it. They can only play their best (and they must be prepared to play their best), the rest (coach recognition) is out of their control. They must be ready to work hard to handle the part they can - which is to do their best.
Regarding academics: this is the one thing that is totally in their control. With this being all in their control, why would the player want to blow his grades when if he did well, this part would be a given and removed from any concern? The better the grades the larger their college pool will be, given the athletic talent is there. The player should continue to work as hard as possible. Once they turn in grades as good as they can, if the talent level it there, there will be a place for them to play.
Let me tell you a short story about my son. In September, after he had his offer from the summer, he completed his one college application and got in to college. He was ready, and signed sealed and delivered come November. At a HS senior year presentation, he was sitting next to a good friend, who like many of his other friends started complaining to him about how lucky he was to only have to do one college application - and how easy it was for him to have to only do one, etc. while they all had to do a dozen or so. Well, I guess he had been hearing this from a lot of his friends, so he very politely turned to this good friend and told them he had heard enough. He told them something like this: "you think it was easy for me because I only had to do one application and got in 'easily'? Well let me tell you how 'easy' it was. While you were sitting on the beach with friends during the summer, I was traveling all around the country attending tons of baseball events, and my summer was three weeks long (it was summer before senior year). During the past 4 school years, while you were spending your Friday and Saturday nights at the movies, or playing video games with friends, I was heading to bed at 10pm or earlier because I had to be on the baseball field at 7:00 a.m. for travel or scout baseball games (or getting up even earlier to drive to them). And I was finishing my homework late on Sunday nights after spending 12 hours or so on the baseball field for scout ball. So, I hope you now understand that it wasn't 'easy' for me." (I must add that he would not have changed any of this for anything.)
Lastly, here is the toughest thing to be prepared for - to be prepared for them to tell you (or somehow let you know) when their baseball road has ended and for you as the parent to accept this and love them unconditionally. Because no matter who they are, the road will end.