Well....you're definitely way way overstocked in that 2.5 gallon. Even with a single fish its very difficult to establish a stable cycle in that size of a tank, and with five you are going to be running into some MAJOR ammonia issues within the next couple of days. For now, make 50-75% partial changes daily and treat with a good conditioner like prime to help battle the ammonia. If you can, at the very least pick up a liquid ammonia test so you can keep an eye on that, though a full kit would be better if you can get your hands on one. Stay away from the test strips, they have a tendency to be inaccurate.
Its always a good idea to take the advice of pet store employees with a grain of salt; they are very poorly educated, and sadly do not give very good advice most of the time, like now. The minimum for a good proper school of tetras is 5-6. Of course any number will school together, but in order for them to thrive and have a quality life, you need a proper school. Remember; surviving is not thriving, there is a difference. Just because they may appear 'fine' does not mean they are truly thriving.
Please return or rehome the Glofish. Like I said, they are not proper tankmates for bettas and do need their own school as well. Its probably trying to school with the tetras because 1: Its lonely and desperate and 2: because you have them in such a small space, there isn't much room TO swim away from them....it may not even be that its schooling with them truly, it may just have nowhere else to swim.
Also like I mentioned before, 5 gallons is not enough space to support a proper school of tetras with or without your betta. While a 5 gallon is a wonderful betta home, when you start looking into other spices of fish it is actually a very small aquarium, much too small for all but a select few species.....and tetras aren't one of them.
If you really want to keep these tetras, you need to get at least a 10 gallon tank and 2-3 more of them. If you can't get anything larger then a 5/5.5 then its best that you return or rehome them.....most stores have a 2 week return policy on fish, so you should be able to take them back without any issues.