The LED will indeed light up, but it's really dim. Too dim for any practical purpose.
If you want a dim but still visible LED use a resistor of 2-5k or so. For a normally bright LED 330Ω is great for a 3.3V output, or 680Ω for a 5V output. Values may need to vary with the LED colour/type, especially blue and white LEDs need a lower value due to the higher V(F).
Yes, of course! This would be one way of doing it, but then again i could aswell use a more suitable resistor.
The objective here isn't to essentially limmit the LEDs illumination, but rather to flash leds as indicators. (With current-limiting for the sake of the Arduino)
I'll tell ya' why this is a BAD IDEA... If one of the students accidently writes a high, you'll potentially fry the LED or the Arduino. Or maybe they do it intentionally to make the LED brighter.
If you want to make it easier, pre-solder some series resistors to the LEDs. When I solder resistors in-line with LEDs I usually heat-shrink over the resistor. If you do that, you might want to use clear heat-shrink so the students can see what's being done.
Plus... It's better & easier to teach the students about diodes & LEDs hand how to use LEDs the "normal way" than to confuse them with a screwy work-around.
I never said i was going to teach the students about this! This would be done at home, by myself, at my desk, with my Arduino, testing the programs that students send me.
(However, due to laziness the resistors are often excluded during the lab-hours at school. I actually thought fo a while that the Arduino somehow limited the current internally, and hence told students that the LEDs would be unecessary. (This is what i was told when i took this course myself 2-3 years ago.) However, after som research i corrected myself and now the students know how and why to pick a current-limiting resistor. No imidiate harm done to neither LEDs nor Arduinos.)