I'm no high note specialist, but I do know that "A" is much harder to slot than high "G".  IMO, "G", is the last "real" note in the trumpet range.  Even when you're tired, you can at least try to belt one out, because there always seems to be something to lean on.  G# also.  But unless you've got the breathing down for upper register playing, then high "A" onwards is a @%@@!!