Hey! So assuming you are using a platform with your school to organise this work experience (such as unifrog), applications submitted by your law firm on that platform usually includes a dress code! If not, you can always drop a professional email to a teacher or the law firm to get a rough idea. As someone who also is doing work experience at a law firm, they asked for smart casual!! Speaking on the concerns regarding who will be there: companies usually have slots for work experience so there is a high chance that you are the only student there. That being said, there could be 1 or 2 other students but you guys may be split up!!