High B6 Natural and C7?

I've been playing the flute for ten years, and have been first chair throughout middle school and high school band (which doesn't mean much, but it's just context for why this is so frustrating) It's my senior year of high school and I still can't hit the high B natural and C way above the scale. The Bb right below the B natural comes out just fine, but right as I move up to those two notes I make that "airy" sound. I know it's not a leak because my friend was able to get those two notes right out, and went as far as to say it was EASIER on my flute, which was news to me. Does anyone have tips on how to overcome this?

I've tried researching blowing techniques and speeding up the air I use as I move up the scale, but I've been coming up to no progress for the past two years. I've tried tightening my embouchure as well, but if I go any tighter I start buzzing like a trumpet!