A lot of teachers advocate raising the tongue to help with high notes.  The rationale is the Bernoulli principle, which you may remember from school.  Usually, it is talked about it in terms of aircraft wings and lift, but there is another part to it that is relevant to us.  The idea is that when a fluid or a gas is traveling through a pipe and reaches a smaller pipe, the speed increases in the smaller pipe:

Higher notes require more air pressure, and the theory is that raising the tongue increases air speed, which raises pressure.  Unfortunately, that is not accurate, as was recently pointed out to me by an engineer from Corning.  Faster air actually results in decreased pressure.  Making things worse, after the air passes the tongue and gets to the front of the mouth, the opening is bigger, so the air slows back down:

Whatever advantage you might get from raising the tongue disappears, but now you have introduced the possibility of blocking the air with the tongue, which is what I used to do (and still do when I play incorrectly).

I learned about this from Arnold Jacobs, the great brass teacher and tuba player.  He was my last teacher and a brilliant man who studied medicine for a hobby and did quite a bit of research at the Chicago School of Medicine using the best brass players in the Chicago Symphony, particularly Bud Herseth.  Mr. Jacobs told me that air pressure and air flow are in constant flux as we move from note to note.  He said that when ascending, air flow decreases but pressure increases; when descending, air flow increases and pressure decreases. No matter what, air pressure never exceeds air flow.  He also said that when there is an insufficient flow of air, which results in insufficient air pressure, a person’s tongue will automatically rise to try to compensate.

Instead of raising the tongue, just blow faster — the more air molecules you put into the mouthpiece, the more the pressure increases in the mouthpiece and at the lips.  Thinking of high notes as farther away from you can really help.  The higher the note is, the farther away it is and the faster you move the air to reach it.