Yes, high school baseball fields are "fair game". I mean, if you see
others using the fields after school, for things like jogging the track, basketball, etc... then .... why would you be different? That is, of course, if you're not planning on being a nuisance in some
other way (like, leaving holes, etc...). Of course, use due discretion, because even though YOU know you'll leave no trace of your presence, some people can be kill-joys. So I still go at low traffic times, and avoid lookie-lous, since our contraptions are admittedly attention-drawing
And to be honest with you, I'd consider YMCA grounds to be similar. Ie.: if
others can just help themselves to the park-like features there, then why not you too? If you're doing no harm, why would you need permission any more so than them? But that's just me.
As you said, you've gotten no response. And when you think of it though, the mere fact that you think you needed to ask them for sanction for this (as opposed to other users, like someone who's going to fly frisbees, for instance, who doesn't ask) merely pre-supposes that something is wrong ...... or damaging or evil etc.... that you had to ask,
to begin with. Otherwise, why would you feel the need to ask, if nothing were wrong with it, and if the grounds were quasi-public already? Or are the grounds off-limits and fenced to all people there?