I think the Arduino is a great way to get into Comp eng. I'm in high school myself (currently gr 11), I'm mostly playing with the Arduino for fun, but I learned most of the stuff I'm practicing with the Arduino from my high schools robotics team (FRC team 3375). I was the only guy on the team that knew C++ and enough electronics to get the thing working, the team was relatively small with 10-12 people working on the mechanical side of it, and me working on the electrical and programming. It was my first time wiring up a robot and I must say, if I played with an Arduino before that time, I would have been able to throw that together quickly.
I learned to keep my wires organized and labeled from playing with the robot, I was so rushed while still learning everything (it was my first electronics project!) that I just connected everything without keeping myself organized... big mistake.
My experience with robotics is very similar to my experience with my Arduino. Whichever one happened first I would have probably learned all the same things. The only difference was the scale.
However now that I have played with fast PLC's (400 mhz PowerPC I think, it was a National Instruments box) with hundreds of inputs and outputs and seemingly unlimited amount of ram and built in multitasking. It's great getting an Arduino and learning to work within constraints of 2 kb of ram and 32 kb of flash.
I cannot thank the school and school board enough for pretty much letting the team play with 10 grand worth of equipment for free. It was defiantly the most fun and rewarding thing I have done in my life so far.