Because you're under 18 it will be the personal care that you aren't allowed to do, or they may let you help with some things, like being the second person for hoisting, but they will tell you what to do.
You are usually allocated one person to help get ready in the morning, but I don't know how that will work if you are under 18, you may do breakfast duty/feeding people etc. Then you tend to be allocated a different guest for each trip, and whether you go on your own depends on where you are/what the guest wants to do. For example, if it is museum most people will be together, or around the same area, but if you go shopping somewhere you may end up alone with a guest. It is good to see it as a mini-holiday, because the guests are on holiday so it is a holiday atmosphere, but just remember to do your work too! The trips are usually pretty good, even if they aren't places you usually go, one I went on was a military museum which I thought would be boring, but it was quite interesting actually (and there were several men in uniform around
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I have been to Sandpipers, Netley and Churchtown twice (although they don't have that anymore, but it was my favourite), so I can't give any specific advice about Jubilee Lodge.
The last night is party night, but the last time I went it wasn't as good as other weeks, but that was because there were hardly any guests there. It's not really posh or anything, but it's nice to wear a sparkly top or something as a change from your worky stuff.
I took a medium-sized bag, I think I wore the same jeans/shorts pretty much all week and just a different top each day, a change of shoes is good because if you do a lot of walking your feet can get very sore. Remember to take things like suncream, and no low cut tops or hot pants
I have shared a room with between 1-3 others, the room sizes vary a lot. At Netley I was in a tiny room with two sets of bunk beds, there were only two of us, but we hardly had enough space so I don't know how four people would have fitted!
For me part of the achievement was doing it at all, the first time I was 17 and hadn't been away from home alone before, and the other times I had had a lot of setbacks with uni and stuff, and faced a lot of anxiety to get through it.
Each time I went I wanted to leave for the first couple of days, so don't be surprised if you feel like that - I would say wait until Monday and then see how you feel, because then you are finding your feet a bit more, and the week seems a lot less to get through by then!
You learn so much about so many people, although obviously that depends on the guests, and how much time you get to spend with each of them. Some of the volunteers will be on gap years from other countries, and they are usually very friendly, as are the staff/other volunteers.
From a practical point of view, take the opportunity to do anything they will let you, because it is great experience - I have talked about it in more than one job interview because of the variety of things I was able to experience.