Cheat to slow down time but keep sims moving at same speed?

Merola's time control clock should do the trick.

Here is the link to ModtheSims where you can find it. If I remember it shows up in buy mode under small electronics. You put it in the Sim's house and click on it to choose which time speed you want:

http://modthesims.info/d/85710

I think 70% worked well for me, but if your Sim is studying you can slow time right down so they're ready for their promotion in the morning :)