It is up to the Head Teacher. I am a Governor at 2 schools and they do not like the radical Union types anymore than we do.
This problem is not just confined to the Armed Forces. They do not like having business in either. At a Confernece two years ago the Personnel Manager of a National chainstore gave us a presentation on what they are looking for from school leavers to enter their management programmes. They had approached 1000 schools and only been allowed into 19!! They would not let in the NHS or similar either.
There are too many 'dead wood' teachers who live in a time-warp and pass on out of date information to the kids. They do not want the real world to creep in and spoil their pension plans. There are, however, also some truly inspirational teachers as well so all is not lost.
On a more positive note the High Schools have now introduced a 1 year Uniformed Services course for post GCSE students. Lately, at the High School I am a Governor at, they had 4 Royal Marines come up for 3 days and 'beast' the kids. Not sure of its worth but the point is NUT does not necessarily talk for the majority, although there is undoubtedly a very left leaning slant within most schools.
What is needed are 'Job Fairs' between GCSE and sixth form where all types of careers are showcased.
I think it's called freedom of speech and choice......