I was a rep and a ski instructor for many years and have not generally experienced this in Austria, Canada or Italy. All of the schools that I have worked with have sorted by ability first, followed by language (if there have been sufficient students to form one-language groups) and age. In Austria all of the instructors speak English (many are from the Netherlands and Scandinavia) and Dutch and Scandinavian teens invariably speak English.
I would think that going with a British TO actually increases the chances that your children will be placed in a group with a wide range of ages. If there are only, say, 16 British children, and if the TO has an agreement with a school that the British children form their own groups, then the age range might well span from 5 to 14 in two groups of eight/nine as the instructors will sort by ability. Even if there are eight 14-year-olds they will not ski together in one group if two are near beginners, three are experts and the other three are somewhere in between - or if they do, they will ski at the pace of the slowest person.
In 2027 the UK the half-term week in mid-February coincides with the same holiday across most of northern Europe. The largest ski school in any Austrian resort will probably be expecting something like 250+ children in that week which means that they can form all sorts of groups specific to age and ability far more easily than if they only have 30 children to organise in a given week.
Another option would be to look at the individual ski schools in afew larger resorts to see if they offer lessons tailored specifically to teens. This is not very common in Europe, but in Whistler, for example, there was a program called "Ride Tribe" (or something similar) aimed at young teens.