* It's a locally used abbreviation: A specific school or district might use "T1" as shorthand for a particular class, track, or level. You'd need to check with the school or district in question to find out what it means.
* It's a typo: Perhaps the intended term was something else entirely.
* It's from a specific program or curriculum: Some specialized programs or curricula might use non-standard designations.
To find out what T1 means, you need to provide more context, such as the school, district, or program where you encountered this term.