Don't tighten your embouchure or you might kill it
There are a few things you can do to help get the high notes out.
1) Make sure you aren't pinching the reed. When students are afraid of the high notes, they tend to tighten their embouchure and it sounds terrible. Keep it relaxed as for the rest of the range.
2) Use a faster air speed - this will change the aperture of the reed and make the higher notes sound more comfortably.
3) Try putting a bit more of the reed in your mouth.
4) Which fingering are you using? Some fingerings are particularly bad on some instruments for obscure reasons like position of tone holes, etc.
In case you don't know the alternate fingerings, here they are:
Eb
x x x | o o x
a c#
Eb
x x x | o o o F
a c#
Eb
x x x | o x x
a c# Bb
I usually find that the last one is the best and has the most pleasing tone, but it will depend on your bassoon so you should try all of them.