Watching kids with bad grades slide by is hard for a parent who makes their own kid toe the line on grades. ( I myself have a "keep it above Bs or no keys" policy...which is slacker by some standards.)
However, (and I really try to focus on this) Baseball is a club sport at the hs. The club leader (Head Coach) sets the rules. It is his club to do with whatever he wants. He can include who he sees fit. He can cut whoever. He can play kids as he sees fit. You may not agree. You have to decide if you want to be in the club and play by his rules or not.
You would hope for some consistency like was talked about by Coach2709. That is the kind of coach you are hoping for. You may not get it.
But, you wont have bosses in life who do things the way you want either. And, I tell my kids that there is a high percentage of things in life you wont want to do, but you will have to do them anyway. And you will have to do them well.
Figure out a way to get along or opt out.
The bad grade kid thing happend on our team last year. An offensively and defensively struggling senior 2.0 kid played over a bad attitude 4.3 gpa (might have been better) player.
Coach played the better attitude 2.0 senior who had given him 4 years. We still won league. Senior went on to play JC.