Check out my playlist on Youtube if your looking for more visual help. I have a segmented run with 3 videos of all 20 levels and timestamps for each level. And a speedrun where i go through all 20 levels in 1 video with no cuts.
Here's the link: https://www.youtube.com/watch?v=sNRVROrDzno&list=PLyFopc6WLn38L6H_wWEukZodyYpbLzKpq

- You can run backwards as fast as you can run forwards. Use this to your advantage so you can see how far she is from you.

- Freya's view distance isn't that far for the first 5 levels (20 meters). After level 5 her view distance goes up to 30 meters.

This is the distance Freya saw me at 20 meters. At 30 meters she can see you from the end of the hallway.

- Opening doors and running will attract Freya if she's close by.

- Don't drain your stamina if you're a good distance from her. Walk when you need to and recover stamina.

- Use the roof to go from one side to the other when needed.

- The main 2 staircases that leads to the roof can be a good area to lose Freya. If she can't see you and you're not running up the stairs making noise she might lose you.

- If you look through the windows in the classrooms on the lower floor you can see across to the other side of the school. Use this to your advantage so you can see where she is and decide where is safe to go next. Same with the main 2 staircases that lead to the roof.

- If you're on the lower floor and running anti-clockwise around the school away from Freya, get in a locker as soon as you run round a corner and are out of sight. If she's close though she will hear you.

- If you're on the lower floor and running clockwise around the school away from Freya, get in a restroom as soon as you run round a corner and are out of sight and close the door and hide behind the door. If she's close though she will hear you. And don't move.
(Might be a good idea to open the restroom doors in advance for later just to save time)

- (When unlocked) Crouching behind the L shaped boards around the school can help avoid Freya. Doesn't work sometimes though. She might shortcut through the courtyard and come up behind you on the lower floor.

- (When unlocked) Use your wolf whistle when needed to get a last minute backup if you're in a tight corner.

- On level 13 Freya will start laying bucket traps. You can use these to your advantage to lure her
to the noise so you can get to a objective that she might be camping.

Hope this helps and feel free to let me know of any more tips to add.