Now you are back in the lab at the time just before you left the first time. No one is freezed this time. You would have to get back to the interim time on another time.
Argh, ask the students. They tend to know everything. Go on, talk to the students. Bravely enough.
You got the tachyon key. Use it on the strange thing-a-me-bobs to make the portal larger.
Don't use the tachyon key now! Use it earlier!
Jump to the portal in the floor. Come back through the small portal (next to the hacienda). Then adjust the portal with the tachyon key.
From this guide's point of view. You are now in the lab for third time.