Saxophones...you need to isolate note names only. Consider making flashcards and/or weekly note name quizzes. Might have a strong student guiding the class through note names, making it appear the class is doing well OR they've memorized the note names in each song but don't really know what all that information means, yet. There's possibly a lack of connection between note names, notes on the staff, and what fingers to use. Isolate that as well, have the kids describe very specifically what fingers to use (D is thumb-1-2-3-4-5-6) etc.
I wouldn't say you're moving "too fast" per se, just not guiding them through all the connections they need to make as beginners.
Also, consider making some kind of easy warm-up to use daily to reinforce foundation concepts (don't skip straight to beginner book lines every day). The book "Pacing for Success" by Darcy Williams has some great warmups and a LOT of great advice for building up skills. https://payhip.com/b/ish5
Hope all this helps! Keep keeping on! Clarinet's a beast but you can do it!