How to best correctly spell this sound?

Remember back in school (or still in school, like me) when someone got called to the office and all the kids in the class made an ‘ooou’ sound with the intonation slowly rising? Yeah… Anyway, I was just thinking about it, and I was wondering what the best way of spelling it would be so other people would understand how it was said, or pronounced?
Also in music, vocalists might use this sound, but people who interpret the lyrics put something like ‘ohh’ which is definitely not the same.

So what do you think the best way of spelling it would be?

If you don’t know the sound I’m talking about, say ‘you’, but without the ‘y’.
If you’re taking Spanish like me, it’s the Spanish ‘u’.