While I understand those are popular urban legends, those statements just don't hold any water.
You can't use a state owned microphone, or a school, to proselytize. That's good. I don't want my local school used to promote Methodism any more than I want the government to have any say in what's in the sermon at my local Methodist church.
You know why it's considered ok to poke fun at Christians? Because they are a hugely powerful majority that dominates a great deal of the government. That's why it happens, and that's why its good that it happens. The powerful should be the target of some mockery. That's good for our system, and it's ultimately good for the church itself.
And what authority does the government have on that? The only authority the government has is to not establish a religion and is specifically forbidden to suppress religious expression. Besides, how is thanking God at graduation proselytizing? How are student lead prayers proselytizing? These aren't urban legends, they are court rulings.