Personally I love the idea that your school is helping with work experience (assuming you're wanting to go into a medical/pharmacology-related field). However, make sure that you are WELL aware of your age, your maturity and what you say. Even as someone who did work experience after my iGCSEs, looking back, there were many things I said that I would have considered as fairly inappropriate, looking back on it now. It wasn't until the summer before my application before I started to realize the things I should be saying/doing. In a nutshell, make sure that you always -
a) Ask for permission before doing anything, it can get annoying but it's better than getting something wrong
b) Think before you talk. Too cliche but very important so you don't **** off patients or people buying supplies
c) Prepare before you start. Understand your responsibilities and what you might need to know. Not asking you to study the pharmacy course, but just know in general what kind of things treat what kind of conditions in general
In terms of clothing, I would opt for a "smart" look, an ironed dress shirt, sleeves rolled up as well as suit trousers and optional black shoes.