Buzzing terminology is abstract and confusing. Tighten up, compress your lips, faster air.... This is one of the few times that I use the pencil trick. Most students would require the use of a quarter at first. Take a quarter and place it between your lips. Then show how lip compression can raise and lower the end of it. Let the student do this as well. After doing this they may have a better understanding of what is going on.
My daughter fell into this group. I let a respected teacher teach her for a while. He is a friend of mine and my kids never listened to me anyway so he taught her. He told me many times what a great sound she had from low g to low c. She never played a d until after I put a quarter in her mouth. Her range went up an octave to 4th space e in a day or 2.
Plus you said that you think the embouchure is formed properly. All elementary and Jr.High students NEED to be checked using a mouthpiece visualizer. A great many children will part their lips slightly as they place the mouthpiece. Most can still play by using mouthpiece pressure to close the gap. If the lips were touching properly they would progress faster and go farther.
I'm old fashioned about all beginners. I treat them like Mendez did. They buzz simple songs before they ever pick up the horn. This helps to build some muscle strength and control without adding bad habits. I know that they want to play and will try however if you insist on 20 - 30 minutes of buzzing a player with 1 week of experience will out play anyone elses' 1 month player. It forces them to learn a closed setting which helps range, tone, and endurance. Let them use buzzing practice after every playing session for a couple of months and you will be shocked at the results.