I first came to Glamorgan Gwent Archaeological Trust (GGAT) near the end of year 10 (first year of my GCSEs). Due to a mix up (at my schools end of organization) I thought that they were looking for a computer aided designer (CAD) when in fact they wanted an archaeologist. The Trust was very helpful for me as they learned about my skill with computers, and used it to their full advantage.
Some of the activities that I did are as follows: I created a flyer for the Oystermouth Castle Community Dig; created a certificate for the children who attended the above named event in two different styles; created a project on the history of my house and my local area; and imported the National Trust boundaries for the Trust to use, as they need to know what finds come from where. I ended my 2 weeks of work experience doing finds washing, the only thing I didn't do on a computer!
After my GCSEs I returned to the Trust for 3 weeks doing voluntary work. Again I did computer-based jobs, this time doing Conservation Area boundaries; I then did some converting and quality improving for several site record drawings, they took ages but I got them done (eventually); then I created signs for the open day in Swansea Museum; my final computer based job was creating a 3d version of their reconstruction drawing of the salt house in Port Eynon. I then did some mass scanning of the drawings that the children did at the open day. My final job was cleaning the Anglo Saxon wall piece that the children put together (one of the many activities at the open day).
During both visits I felt very welcome and I'm sure that they found me very helpful. Whether you have plans for archaeology or if it's just a slight interest I would strongly recommend coming here for your work experience, as whenever I had a doubt about a job I was given they were happy to help and it is great fun!