If I remember correctly. There is a glitch with the punishment system in Generations where a parent can autonomously ground their child from leaving the house. If you don't let the child off the hook then they would be stuck in sneak mode forever and the option never shows up to let them off the hook once it's too late. I guess if the teen attempts to sneak out before being let off the hook is when the glitch becomes permanent. Mods are the only way I know to fix it but maybe that isn't necessary. I remember having that happen to two of my teen Sims and nothing fixed it, not even when they aged up.
DebugEnabler (Click on Sim > NRass > DebugEnabler > Reset Walk)
http://nraas.wikispaces.com/DebugEnabler
OR
Locomotion Changer (Clear Walkstyle)
http://modthesims.info/download.php?t=385612
And here's a mod that also may be of use.
User-Directed Scolding + Other Punishment Tweaks
http://modthesims.info/download.php?t=533906
