teenager ok so i have been playing and decided i want my family to stick around the way it is - so my teen drank the elixir of life - so when she was back at 15 days she had another drink of it and it didnt change from 15 days...?? also - will getting a job for the teen mess up her report card?
Not as long as she can manage to work and keep up with her homework, and still get to school in a good mood every day.
Make sure that her aspiration level is at least gold when she drinks the elixir or it won't work properly. (It can even make her age faster. Which is good if you are trying to quickly age a Sim, but bad if you want them in that stage longer.) If they get a job, it takes about 3 hours out of their evening time. If they are already tired or lacking fun, it can make that worse (so that they may not want to do things like their homework.) However, if they are in private school (which allows them to come home less lacking in fun), it's a little easier to balance their fun, social, homework and job.
15 days is the full length of the teen life stage. So you can't get more than that. You'd have to wait until she's aged 3 days (12 days left) and then drink it again to bump it back to 15 days. And unless your teen wants a job (like if she's a Fortune Sim) I kind of feel teen jobs are more trouble than they're worth. They don't pay a lot so unless your family desperately needs an extra $45-125 a day, I don't feel like they're worth it. It doesn't leave a lot of time for anything but sleep, school, a little fun, work, eat, homework, sleep again. Whereas, if they don't have a job, they have a lot of time after school to satisfy fun, do their homework, then skill build or cook dinner for the family, do some cleaning, take care of younger siblings, etc.