Both "I'm in university" and "I'm at university" are commonly used and generally accepted phrases. However, the choice between the two can depend on the context and regional preferences.
"I'm in university" is commonly used in British English and other English-speaking countries outside of North America. In this usage, "in" implies being enrolled as a student and actively pursuing an education within the university system. For example, a student might say, "I'm in university studying biology."
On the other hand, "I'm at university" is more commonly used in North American English. "At" suggests physical presence or location, emphasizing the idea of being on the university campus or attending classes. For instance, a student might say, "I'm at university taking a lecture."
Ultimately, both phrases are acceptable, and the choice may depend on regional conventions or personal preference.