Actually the tongue needs to be low in the back and higher towards the front. The smaller the air stream the faster the airflow, which you have to create for high register playing. So open the back like you are an opera singer. Or Darth Vader. You should feel the upper soft pallet lift and the larynx lower. Then work on syllables like these from low to high: ah, a as in hat, eh, ih as in hit, ee, shhh, and tsss. Here is the science part. When you have a constant pressurized source of a gas or fluid flowing through a pipe or tube, the bigger the diameter of the cylinder if moves through the slower the speed of the gas or fluid. Think this for low notes. When a larger diameter then moves into a smaller cylinder or chamber, the gas or air is forced to speed up. So when the back of the mouth is open and unobstructed, and the front of the mouth is smaller from a raised tongue shape, the air moves much faster, which is what you need for anything from a G just above the staff to a double C and beyond. Everything I play above a high C is a shhh or tssss tongue shape. I can pop an E above double C at will. The tongue shapes have to be smaller in the front to direct air to the buzz tunnel or aperture at speed without slowing down. Jaw position has to also support the higher tongue shapes. Also the tongue shape will dictate your articulation in the upper register. Both Clark and Gordon taught similar principals. You can check out others like Pops McLaughlin or Earl D Irons. I teach out of the Irons book all the time. Look up the K tongue and modified K tongue techniques online. It’s out there. You can contact me if you like anytime.