The electrical outlets on the outside of the school building are not there for the public to use. They are there for the use of the school and/or who ever the school gives permission to use them.Yah see, but that doesn't apply here. I don't believe he unlawfully took it or that it deprived any other person of any property, which are the two points of that law.
I'm looking for case law now, give me a night. To me this is akin to going to a public park and flushing the toilet. About the same costs and both are public resources open and available for use by the public. The outlets just like the toilets are made available for the public to use. That's the whole reason they are there and it explains why the school is not pressing charges.
Bathrooms at a public park are there for the specific use of the the park visitors. And if the park closes at night? Then the bathrooms are not open to the public either.
I live in a city where some businesses, municipal offices, etc. supply outlets (in the parking areas) so people can plug in their vehicle's block heaters (hey, the high the other day was like -6! Not counting windchill). Those outlets/plugs were in place before electric cars, however, it could be assumed that plugging in an electric car would be permissible. However, it would not be ok to plug in an extension cord running to your home so you could have electricity to power your big screen TV!