(at around 40 mins) As the Scoobies return to the school for the second time looking for Buffy, it is obvious that while standing in front of the school, it's clearly the same shot as the first time they stand in front of the school. This is particularly noticeable because you can see Buffy, who's inside the building at the time, also standing in the group.
When the 1955 yearbook falls from the bookshelf in Principal Snyder's office, it's open, but when Buffy goes to pick it up, it's closed.
There is no way that any authority, or pest control service, would allow the snakes to be left in the School canteen overnight. There are multiple ways in which they could have escaped and spread to other parts of the school, or outside, such as the visible air vents.
The murder-suicide of the student and teacher happened in 1955, but while the iconic "I Only Have Eyes For You" was written in the 1930s and covered by many groups thereafter, the student is listening to the 1959 recording by The Flamingos.
About ten minutes in, after Buffy has had the vision in class, and she and Xander walk through a doorway, a reflection of the boom mic is clearly visible.