Are you really sure you can't do skirts/ dresses? It all depends on the length. If it's the right length a skirt/ dress can be so much more flattering than trousers.
I bet you aren't actually frumpy but what you need to think about is something simple, classic and comfortable. Decent underwear underneath (I'm not talking girdles or corsets but just a decent fitting bra and knickers that won't be uncomfortable or give you a VPL) and shapewear tights will give you a good silhouette.
Have a look at something like the Isabella Oliver 365 classic wrap dress in black
www.isabellaoliver.com/womens-clothing/uk/600/dresses/DR432.html
The curved shift dress and the zig zag dress are lovely as well although they are a bit more pricey. The beauty of the wrap dress is that you can wear it to other interviews, dress it up for an evening out or dress it down over jeans with boots or flats.
Pair with simple (and comfortable!) mid heel shoes - perhaps in black patent. M&S usually do a fairly good range of simple court shoes at very reasonable prices. If you keep them only for interviews then they'll last you a good while. Use a smart mid-size leather bag. Big enough to put all the essentials in but not so big that you're struggling to carry it and it is difficult to manouveur when you're shaking hands with interviewers etc. Keep the contents to the bare minimum - hair brush, powders, lipstick, tissues for a quick freshen up if you need it.
Always take extra copies of your CV and cover letter. It's good to refer to if necessary and you'll be amazed how may interviewers forget to have a copy of it when they start the interview. Makes you look super-organised!
You may also find that you have to lead the interview to an extent. Not many people gets training in interviewing people so it can be difficult to sell yourself to them if they are not asking suitable questions. Although you don't want to talk too much make sure that you have the opportunity to mention what you think is relevant. Give examples when replying to questions as to how you achieved a certain objective etc or how you would/ have dealt with a particular situation.
Don't overdo the jewellery or the make-up either. Keep both simple. Make sure nails are manicured with a neutral varnish. Have a shower the night before to do all the essentials such as shaving etc so you're not rushing around on the day of the interview doing all these things. Get an early night!
Leave plenty of time to get to the interview. If you get there early enough, nip to the loo to do a final check etc. Drink water during the interview rather than tea or coffee - if it spills for any reason less of a disaster plus it's more refreshing when you are talking quite a lot.
Think about your posture - practice in front of the mirror standing up tall - you'll be amazed at the difference. If you enter the room walking tall and confidently, with a smile on your face and a firm handshake, that will make a huge difference. First impressions really do count in an interview situation. Also think about your body language in the interview and making eye contact with the interviewers. Sitting straight and not fidgeting with hands etc is important. Keep your posture open to the interviewers so Make a list of questions you want to ask them - lots of interviews finish on this note. If you have some pertinent questions to ask them then it shows you have done your groundwork. e.g. how do they see the role developing over the next 3 years etc.
Finally, have confidence in yourself and your abilities. I bet you are not frumpy at all. It must be daunting to be going to an interview after so long but as long you have done all the preparation (reading notes, job spec, research about the school, practiced replies to various interview questions) you will do a cracking job! Good luck!!!!