How can I provide security for an in-ground swimming pool?

Work with your local sub station for a police drive by nightly, add a higher fence. An aluminum pool fence (http://www.ifenceusa.com/pool.php) has fewer "footholds" than a chain link fence, harder to climb.

Seconding easily confused above, I believe lockable, alarmed pool covers are the traditional route here.

I can't help with the tech, but do you have a good solid lockable pool cover, in addition to the fence and the rest?

If your cameras aren't telling you anything about who these kids are, you need a better camera setup (otherwise why have them?). If they are, then your community needs to at least ascertain if they are kids who live there, or friends of kids who live there. You could take pictures around to the residents and see if they can ID the kids. Yes, teenagers are gonna teenager, but they're not like wildlife; you can prosecute them for continued trespassing. The other options for locking up/security are good too, but right now these kids have decided that no one is really paying attention, so why not keep doing it?

As a teenager, I had a pool. But that didn't stop me from colluding with my friends to get together in the wee hours to skinny dip in someone else's pool, uninvited. It's what we did and the harder they made it to sneak in, the sweeter the reward. Barbed wired, dogs and armed guards dispatched were all part of the fun. To this, I would make sure you're in compliance with your insurance and set up a motion activated spotlight/camera to later shame or prosecute the offenders. Anything that could physically injure an uninvited guest will result in an unpleasant courtroom experience for the condo board and/or persons responsible for implementing it. Not to mention that it detracts from the ascetics of the pool area.

You can put spikes on top of the fence to deter people, pressure-sensitive trip wires around the perimeter to deter teenaged-sized bodies, etc. Or a higher fence. 6 feet is trivially easy for a teenager to climb over.

Some sort of paint of non-setting goop on the fence so the little teenage clothes get all crappy?

I'd second the "avoid more dangerous climbs". Had a friend who did a number of her hand climbing over the barbed wire (as a non-drunk teenager). Injury potential isn't a deterrent. I would second working with local police, and probably your condo insurance agent (since they have a heavily vested interest in preventing things .. ) for solutions/ideas as well.

Seconding k5.user on working with your condo insurance agent and the condo board. A new, appropriate, and vertically challenging "proper pool" fence may lower your insurance rates in the long run, making the dent in condo savings or the bite of an assessment to pay for it much more palatable.