I am not sure if this is even measurable anymore. A lot of things that happened in the past were not considered bullying. Even really serious stuff.

Additionally, especially when looking at anecdotal evidence, how respectful kids treat each other can be different per school and even for each class.

For a few things we have semi-reliable statistics. For example in Germany the number of knives that are regularly taken from students has significantly increased. Also the number of times schools called the police because they needed help getting a situation under control. But this could also mean there are more extreme single cases. It doesn’t necessarily mean the overall atmosphere is more aggressive.