As I practice and play the trumpet, I work for clear notes, yet I often get "interference" in playing notes that reminds me of static in a radio transmission (like a sputtering). I presume it has to do with my lips, so how do I alleviate it and get a clear note? Also, that high C (above middle C) takes some work! What are some tips to getting it?
Ah yes...the dreaded "fizzle" or "interference". Now, without seeing your embouchure or hearing you play I can only do so much to help.
Essentially most issues with tone production come from lip placement and air flow. Every now and then the fizzle can be caused by a leaky a spit valve cork or other anomaly within the structure of the horn, so be sure to rule that out.
The best exercise I can tell you to do is long tones and eliminate all vibrato. You want a tone and stream of air so solid you could rest a plate on it without it wavering. Play tones until you feel the air beginning to give out and then rest for about 10 seconds or so and then repeat on the same note or a different note. A nice mf dynamic is good. Concentrate on the sound and if you feel or hear the fizzle, evaluate, correct, and play again. Every face is different and each embouchure that comes with it. Ultimately the reason the fizzle starts is there is not enough air to support the embouchure for the note you are attempting to play.
As for your range issues. Again, I can't see you but realize that increasing range is like any muscle building you would do. It takes time and small steps to do it right and without compromising sound quality. High C is that note that all high school students long to hit consistently over and over. It also seems to be the note where most embouchures have to adjust slightly to change into the upper register above that. The concept behind any note that is higher than another is the same throughout the horn. IT requires a little more air and a solid hold on the corners of the lips to keep that note up there. Oddly, every instructor out there has a different approach and to be fair...not all approaches work. There are many exercises you can do to help you. One of the weirdest and best I have found is to work on the lower pedal register. It seems counter productive to do that, but the lower register requires more air and a control to get an actual note to speak than you might imagine. Secondly, the long tones you are working on will help get the muscles used to getting up there...but take it slow. Half steps at first is your best option and don't try to blow the roof off the place. Louder does not equal higher. Lastly, I will suggest lip slurs. The key to increased range lip slurs is for you to start on the upper note first. If you're slurring second line G to 3rd space C, start on C and go down to G and bounce back up to C a few times and always end on the upper note you started on. Why? To get that upper register feeling throughout the range of the horn so you know those notes are there, wouldn't it make sense that you start there and adjust downward as opposed to upward. Over time, you upper register embouchure will take over (so to speak) and that will be your basic embouchure allowing you a bigger range of muscle adjustment.
I hope any of this helps and good luck in your continued endeavors on the greatest instrument ever.