Depending on the design of plumbing infrastructure, it's possible that the authorities had to arrange for pipes to be dug up, since they didn't have access to the premises where the localized mains are. Similar would be the case with electricity, except less effort would be involved, since wires are typically run along utility poles rather than buried underground.

I dont think they wanted a load of young kids tired and jumpy and spooked with out water, food and light at night seeing as they were all armed with real guns and bullets.

Also, the authorities might've been hoping that Bache would come to and talk the kids out of the school. It appears they cut the water and electricity around the same time or shortly after Bache died.

They were, after all, children. They also believed it measured out to a hostage situation. They were undoubtedly thinking that negotiation was the best tactic and only when Morlan proved unreasonable and unstable after hearing about the General's death did they resort to tougher measures.
Negotiation is a tough deal and you can't squeeze too hard or the firecracker will blow your hand off. It's best to treat a firecracker with a loose grip.
Speak louder, Mr. Hart! Fill the room with your intelligence!