Could the mentor do a little more?Probably.
Are you equally being a bit precious? Yes.
Ultimately, you're there on work experience and you start your training in September. I would imagine the reason school have asked you to come in for a few days is so you can get a feel for the school and how things work, where things are etc. Or they've asked you to get more experience of secondary before the course starts (which is par for the course i'm afraid)
I career changed into teaching and based on your posts, you appear to have a touch of the attitude of other career changers who found the transfer difficult.
Trainees should never be accepting of being shat on, but they do need to be aware of the reality of school life and avoid being oversensitive or overcritical.
E.g. I can go 3 days without seeing some members of my department. Nobody is rude. People are busy. I had lunch with someone on work experience on 2 days of the week they were in, other days different colleagues were in the staff area. Unfortunately school life doesn't stop because there's someone in for a few days and once you're in school fully you'll realise that. Equally, your mentor will probably have lots of directed tasks to do in their gained time and so can't drop them to sit down and chat/go through everything/be on call all week. Whether you are being awkward and expecting too much or the mentor, well that could be either side. Equally, I've met a not insignificant number of trainees who seem to think that unless their mentor is fussing over them and dropping everything for them that they're being ignored and neglected . I'm afraid that there is a fair chance you're taking part and parcel of school life to be a sign people are rude and unhelpful.
I'd say you need to clarify the purpose of your time in school. Is it to familiarise yourself with the school? Is it to get to know the department? Is it to spend some more time shadowing classes? Is it formal induction?
Your course will set out the expectations for mentors and trainees, including meeting times, observations etc. That will kick in from september. They'll also be able to explain how and when they are free on top of that (and remember you're joining a department so support is more than judt the mentor). Clarity of expectations is key for an effective mentor/mentor relationships