Hello Anoushkev,
I am sure others here know more than I do, but from what I gather if you sent in an application for the Open Work Permit (OWP) along with your package, you are under what is considered 'implied status'. This allows you to stay in the country without an extension of a visa... however it does not allow you to extend work or study permits. So you would be able to stay in the country, but not continue studying past May 2015.
As for leaving the country, because it's before your Study Permit runs out, it might be okay... but I would personally advise against it. Technically, there is no guarantee you'd be let back in to Canada on your return. I think it even says on the permit "does not guarantee re-entry".
(If that's wrong, omniscient and wise forum folks, please correct me!)