Yeah. I've changed my mindset from when this originally broke. This was an integral part of their scouting. The players obviously prepare with it, so it's clearly known by everyone.
A high school team could do this with tools they already have. Modern scouting already works this way. A HS assistant coach watches hudl and plugs the plays and whatever information is being tracked into a database. The computer does all the checking for correlations and outputs whatever and however you want it. All you would do extra is input the various signs.
Responding to other posts as I catch up:
1) You couldn't do the same thing with public information. By rule, all-22 can't show the visitor sideline and home teams hide their own. You could make an effort to gather as much as possible from broadcasts, but you'd end up with more uncertainty and holes than useful information.
2) A team can't simply "change signals" to combat this. Teams might have different sets of signals, but if your sideline is being recorded every play across several games, the opponent is going to have all of them. And you can't like go in at halftime and completely change with no prep or practice.