Extra working hours taken by volunteer

[I8haggis4T]

I work in an independent school where lots of volunteers come and go to get work experience before applying for teacher training. As I am always looking for extra income (they say they do not have the funding to give me any extra days), I am always upset if they cover extra hours with one of these volunteers as I work extremely hard for my meagre wage (not to mention the fact that I actually provide the work experience training for these individuals). I believe it is just penny-pinching; it would not cost them that much to pay me for an extra week each year to cover staff away on trips, but it means so much to me. It's a great pity that I do not have the guts to go off and do the year teacher training course myself, but my DS and DD are at critical exam year stages and the teacher training is so intensive that I do not wish to put myself before supporting them (and once they are both finished I believe I am too old to start a new career).
I would be very grateful if anyone had any knowledge of employment law with regards to volunteers taking away work from a paid employee.

Are they taking hours off you or is it overtime?

But they have not cut your hours, so they are not taking from you.

It's not taking hours away from you if it's extra hours that you are not contractually entitled to. What does your contract say, does it give any guarantee that extra hours will be allocated to you?

The school has a tight budget so I can understand why it uses volunteers when it can. If you want extra income maybe you could try looking for another part time job.

I would think your hours/pay have not changed and the school allow volunteers to cover certain activities only under supervision of paid staff. Just because you are available during that same period does not mean they have to offer you first refusal. Could you not do the teacher training alongside your pt role ?

If you have two children at "critical exam stages" I assume they are doing GCSEs and A levels this year so find the assumption that you'll be too old in, what, 3 years, a bit strange.

You are all right, they are not obliged to give me any extra hours as part of my contract, although when they originally took me on as part of a job share they verbally promised there would be additional hours during the year and they would appreciate me covering it (a verbal contract in Scots' Law is binding btw). Infact they almost did not allow me to apply for a job that came up with an extra day as they said (and I have it in writing) that they liked the fact that I was able to cover whenever they needed me to! I have looked for other work but as so many T.A.s in the state sector were made redundant here due to cut backs, it has proved impossible. I have tried two jobs a few years ago before the glut in the market of redundant T.A.s, but I was reminded that contractually I have to do inservice days outwith my usual working days and the second job did not allow that.
Also, my DD will be out of school in 4 years (Scottish system - my nickname is a give away) I will, sadly, be drawing a pension in less years than I would really like to admit! Would it really be wise in this day and age to give up a paid job to do a course I may not have the courage/ability to finish and then face a uncertain job market competing against bright young things (yes there are age descrimination laws, but how easy is it to prove?).
Back to my original point, the school legally needs the cover filled, they are not so hard up as not to have splashed out recently on numerous things (I could give a really frivolous example but I would give the school away!) and I get paid less than the cost of a cleaner (I know as I can't afford one!). I'm sure unpaid law interns were being criticised in the news last year for taking away employment opportunities from regular workers, I just wanted to know if this was due to a point in law, so that I could back up my case at work.

Oh and I did look into doing teacher training as well as my job. There is a course that does one evening a week, but then a block of work placements during the day which was useless as I cannot get time off during term time. The Open University no longer runs a course in Scotland. There used to be a distance learning course over two years which would have been great, but it has stopped now.