Don't take this wrong. It's honesty time ...
What makes you think you can play at Belmont Hill? They're perennial ISL baseball powerhouse. They recruit players. Are they recruiting you? If you're not being recruited by BH your chances of playing are about as good as walking on at college in the future.
Your goal is to play D1? Why Thayer? Only two players from Thayer are playing mid major D1.
I know Dexter was undefeated last year. But I don't know much about them.
Is your parents willingness to pay for private school based on having the potential to play D1 ball? Or is it academics? How are the academics at your high school? How will you parents feel if they spend the money and you end up playing D3?
Keep in mind the players from these programs who do play college ball at whatever level are probably getting there through their summer teams.
Given the schools you've mentioned why are you considering St Johns Prep?
Do you know how many players from MA go D1 each year? How do you match up with these players?