First, reed thickness is not one thing: The traditional Vandoren reeds are stiffer that their V12 reeds. Approximately, thickness 3 Traditional equals thickness 3 1/2 from the V12 variety.
Second, ligature placement and tightness is also a big issue. Amateurs tend to put the ligature too close to the the tip of the mouthpiece, and/or tighten it too much, with devastating consequences for pitch and tone quality.
The shape of the mouthpiece is also of great importance when it comes to tonal stability and pitch. People always talk about the reeds as if that’s the whole story. Also, the embochure is very important, of course. None of these three are independent of the other two.
I suggest the band teacher try the mouthpiece/reed combo for himself. If he can play in tune, it’s your daughter’s embochure that needs a workee. If not, I suggest that she changes to a more open mouthpiece: Vandoren B45 is open, and allows great freedom when it comes to adjust pitch during playing. It’s heavier to blow, and professionals always uses a thinner reed when using an open mouthpiece.
Most intermediate level performers will do well using the mouthpiece/reed combo B45-2 1/2 or 3 Traditional Vandoren or B45-3 or 3 1/2 V12 Vandoren. Why an amateur should want to play on a 4 1/2 reed beats me totally!
And be careful with the ligature!