You'll need to prepare some location cards (included below). I made this for around 35 to 40 students in a class, but of course you can just adjust the number of cards to fit the number of students. Lamination is advised.
Each student gets a location card that they should try and keep secret from It/the Oni. At the start of every round, the student who is It will call out a location card after they spend around 10 or so seconds patrolling around the classroom (the other students remain seated). All the students who has the card It called out must then stand up. The student standing up who is closest to It must then play rock-paper-scissors with It. If It wins, then that's tag and the other student is now It. If It loses, then It will continue to be It for another round.
For the first round, I suggest you demonstate how it's done by being It.
The "Safe!" card is self-explanatory. If a student has that card, then they are "out-of-reach" for that round.
After the rock-scissors-paper phase, the students must switch cards with 1 other student adjacent to them. They cannot switch with the same student every round and must switch with someone else. However, after every other round, they can switch with the student they switched with before. For example: Student A switched with Student B during round 1. In round 2, Student A cannot switch with Student B again, so they must switch with Student C or Student D. But in round 3, Student A can once again switch with Student B.
While not necessary part of the rules, I think students should call out to It every round if only to have them practice English. Just a basic "Where are you now?" will suffice, and It can simply answer with "I'm [location]."
Note that I haven't actually tried this lesson yet but I felt lowkey proud of myself for coming up with this and just wanted to share. You can probably adapt this into a Halloween game.
Edit: finally got the chance to do this activity. It most definitely does not take up 50 minutes. At best, it lasts for 35 minutes.
Edit 2: added a Halloween version where instead of tag, it's a Zombie infection game. No Oni/Monster. Instead, you start with 1 zombie (usually you, the ALT) and try to infect the students by winning in Janken. Unlike with Tag, the Zombie doesn't get 'revived' unless they lose against a student. You only have the students change the cards at the end of a round, which means the Zombies all have to say different locations (unless you don't mind them ganging up on one student. That can be fun too, in my experience.) Usually it's nigh impossible to convert everyone, and Zombies get wiped out if you don't have enough, so don't be scared to start out with more Zombies if need be.