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If you bring your tongue up, the notes will come out sharp.

You may need a stronger or better reed. make sure the heart of the wood is shaped properly, and see if it needs to be filed down a bit. Tighten your embechure for the higher notes so the reed has vibrates less and make sure your ligature isn't to lose. Other then that just sit up straight breathe deep and think of the sound coming out of the top of your head it'll help raise it so it isn't flat

This was one of my answered questions previously before I went to a summer camp at Stetson University.
1: Don't play them to loud, if you try to make them louder, they get squirrley and/or abnoxious
2:Make sure you reed is right, not to hard or too soft, not waterlogged, and especially in the right position (My [free] private clarinet teacher gave me a trick to play your clarinet to the side, like, hold a middle "G" on each side to see if your reed is crooked)
3:Sit with good posture, I learned that the hard way!
4:A tuner is a good tool to have, usually your band director should have one, just visit, and play concert pitches, and if it cirles around, to the left it's flat, and vice versa.
5: Don't breath through your lungs, you must have good breath support from you DIAPHRAM! Just like in singing! xD
6: Make sure the holes are covered all the way, and the correct way, people make those dumb 1st year mistakes all the time!
7: Make sure the ligature isn't too tight or lose, and make sure that it's not crooked or bent (Rodner ligatures are the best)
8: Your embouchure is soooooo important!! Don't "smile" or use too much or too little mouthpeice. Your bottom lip cannot be soft or be in/out too far! That will make a big difference
9: Last but not least, practise, try two/three octave scales (if not allready) Chromatics are the best excersixes, and long tones with the high notes are good too!

I learned most of these the hard way and can now do a three octave chromatic up to the highest "G" easily. Most of these tips are from very good instructors and professors, so take my word for it, it works!!

Good luck!