First off, you've provided no study or actual evidence other than your anecdotal. So even that is iffy. Why? Because you're relying on the assumption that school disciplinary problems have actually gotten worse. This is based off nothing but your perception. So, what are other reasons you may have this perception.
1. When you're younger you're not as aware of these things on a local or national scale
2. People have a tendancy at times to think back more fondly and favorably upon the past then reality
3. The increase of news proliferation has caused an increase of coverage of these issues, making an illusion that more instances of problems are happening then actually true
4. The school you went to was in a different local than most of those having issues currently, and as such comparing say an inner city school now to a suburban school 30 years ago gives you a false impression as its not comparing the same general school condition in both time periods
All of which are potential options for the perception that schools have more disciplinary problems over the past 30 years other than simply it actually being an increased problem.
But now lets actually assume your assumption is correct, that its gotten worse over the past 30 years. There's absolutely no way to come to a single conclussion as to why. Some other options other than the decline of corporal punishment...
1. Increase in populations leading to larger schools leading to less ability by administrators to keep control of everything
2. More lax, uninterested, single parents all around leading to kids less disciplined in a general sense (mind you, this could include parents who routinely hit their kids, but that's the only form of parenting that's done at all)
3. A general shift in entertainment geared towards kids over the years highlighting the rebel/trouble maker types as the "Cooler" type of kid
4. Further restrictions on schools in regards to how they themselves can discipline and control kids, thus allowing things to get further out of control
And I could go on I'm sure if I felt like thinking about it longer.
Sorry, your one liner little comment is nothing but a baseless opinion founded in assumption with no reason anyone should take it seriously.