YANBU.
Even DS3's school, which is the parent unfriendly school I've come across, had induction days where the DC were taken around the school/shown classrooms while we listened to the adult stuff. It really is a bit ridiculous to do it in the evenings when it's obvious the DC will need looking after but cannot be taken.
I struggled with no support network for years with 4 DC (not SP but DH who worked away/to very late in the evenings). Thankfully DD is now an adult so can babysit on the very rare occasion it is necessary but it was a nightmare for years when they were all younger.
It really is hard to just 'find' yourself a support network. It gives me the rage when people, who probably have a great support network themselves, can't imagine how people who don't are in that situation.
Some people are unlucky enough to not have family members/friends living within nearby or who actually give enough of a shit to help out.
Some people find it very hard to make friends, may be shy/socially anxious, and you need a good friend to trust them to look after your DC.
Some people actually cannot afford to pay for babysitters, especially if they are a single parent on a low income living hand to mouth, who can't 'magic' up £30+ for a few hours ad hoc childcare for something which could have been organised better.
Some people don't want to leave their small DCs with strangers.
I would call the school, explain your predicament and ask to come in at a another mutually convenient time (when DC at pre-school?).