Say they reject you during early admission, can you reapply at the regular decision if for example your SAT/ACT score improved?
If you are deferred, you are automatically placed in the regular decision pool. If you are rejected, you cannot reapply.
No you can’t. And dang, so sorry for the rejection! But, I doubt your test scores can improve so much that it could change anything. Penn usually rejects a small percentage of its applicants (less than they defer), so if you were flat out rejected it pretty much means that there is nothing you could do between then and March that would make them want to accept you. Sorry for the harsh truth, but best of luck!
For the top schools that are not known for rejecting many applicants (ie. everyone but Stanford), a rejection in the early round is generally the result of multiple factors. It’s extremely unlikely that the rejection was the mere result of a poor SAT/ACT score (something outside of the necessary range would be kept on the table in the event that the student could improve his scores by a reasonable margin), but the result of an unimpressive application.
In short, if you get rejected from a school in the early round (and your SAT scores aren’t too far from the mean), it’s quite unlikely that better SAT/ACT scores would have gotten you accepted in any round.
@Lanie49 I’m still in grade 11, I was just wondering since I probably won’t be able to write the ACT the first time until early next year.
Why can’t you take the ACT before 2016? The ACT is offered throughout the year. If you’re not in the US, check to see when & where it’s offered before 2016. US unis also take the SAT – that’s another alternative for you – again, with multiple sittings throughout the year.
Then you have plenty time to improve! Good luck 
Because I won’t be able to start studying regularly until maybe April.
^but you can still take it in June then…