In my opinion:-
Lack of W&CG/Perf access: knowledge that you are a licenced pilot, and that 2-up with partial tanks from a known and fairly long runway, they're irrelevant anyhow.
No mixture below 3000ft: This is a common UK view: some aircraft (I have a 1969 Condor which actually placards this) do recommend this approach, but in reality it's just burning fuel for no good reason in a long cruise. Also nervousness of low ability student pilots damaging the engine through misusing it. If you understand it, use the mixture, it's what it's for.
Dipping tank: Probably because they know the aeroplane well enough, are flying relatively short sorties, and are always flying with good fuel reserves. When I used to part-own a Warrior II and flew it regularly, I was pretty good at telling within 20 litres or so how much fuel was in the tanks by sight.
An obvious question for you - if you are a reasonably experienced pilot, why not look to local syndicates, rather than pay commercial rates in a school which will be very restrictive of aircraft access anyhow.
G