Yeah, what @slim said is gold. Also check out this:
Everybody struggles with high notes, ESPECIALLY at the beginning.
-7, -8 are the most frequent ones that REALLY trip people up, but anything above 6 can prove problematic!
Remember to be patient with yourself. This may take some time…
These are the tips for a getting good clean high notes.
1.) Let gravity lower your jaw so that it’s totally relaxed and in a nice “aw” vowel shape, your teeth should be far apart from each other
2.) Keep the back of your tongue down, like it is when you yawn. Make sure it’s not in the “Kk” zone (how your tongue is when you say a “Kk” consonant.)
3.) Nice steady, GENTLE, airflow. Most beginners (and many intermediate and advanced players) use WAY to much pressure.
Hope that helps! 
Aloha,
Luke