First of all - well done for getting the interview. You've been shortlisted so presumably they are happy that you have the skills they are looking for on paper so go in to the interview feeling confident. Was there a job description when you applied for the role? If so, have a look at the sort of tasks listed and try to think of examples of similar tasks you have undertaken.
A lot of job descriptions include a person specification which lists the skills, knowledge, behaviours etc. required for the role. If there is one, then it's likely that the interview questions will be based around this so again, try to think of examples that could demonstrate some of the skills, knowledge etc. listed. Questions are often in the form of "tell us about a time when you had to manage several competing priorities" or " tell us about a time when you had to deal with a difficult customer" etc. When I'm interviewing, I don't mind if the example given is fairly old (particularly if someone hasn't been working for a while) or if the example relates to something outside work, so long as it's relevant to the question.
If you're asked about something that you can't do or need a little development in be honest about it. For example, it you're asked about Excel, you could say something like "I'm more familiar with MS Word but I'd be very happy to develop my skills in excel if that was required for the role - I'm always keen to learn new things".
Focus on what you can do for your employer rather than how convenient the job might be for you re school hours etc. For example, if they ask if you have any questions at the end of the interview show an interest in the school, e.g. perhaps asks how many children attend, how support staff work with the teaching staff, what would a typical working day be like, what training opportunities are there etc. rather than asking about hours, flexible working etc.
Regarding what to wear, what you suggest sounds fine - don't get too obsessed about it. To be honest, I barely notice what people are wearing when I'm interviewing unless it's something particularly striking or clearly inappropriate.