There’s been quite a bit of speculation on whether or not Jackson would have enough time to be on the field in the lacrosse game and menace Stiles and Derek at the pool.
Here is the chronology of that episode (times are in-show time minus commercials)
Stiles' Text Sent
23:54 Stiles Texts Allison “Nothing Here” Erica walks in on him.
Boyd Takes out Abomination
23:57 – 24:47 Lacrosse game – Boyd takes down the Abomination on the field.
Erica Brings Stiles to Derek
24:50 Erica brings Stiles to Derek at the Pool
1:04 Left in the Game
25:15 There is 1:04 left on the clock in the lacrosse game.
Allison gets Text
25:19 Allison gets Stiles’ text
Final Play Begins
25:36 The final play of the game begins
Scott Breaks Leg
25:52 Scott breaks his leg.
Stiles begins Kanima Description
26:36 Stiles begins telling Derek about the kanima
Kanima Appears
27:04 Kanima appears in Pool area.
I think the key to the timing here is the text from Stiles to Allison. It took about 2 minutes of in-show time to arrive.
Also there is a minute left in the game when the final play begins and about a minute after that Scott breaks his leg and the game ends.
These events tell me that in-show time is closely following a real time clock.
Remembering that all uninjured players had to be on the field until the end of the game and if we assume that all events on the show are happening in near real time, Jackson would have approximately 30 seconds to transform and make it from the lacrosse field to the pool area.
Verdict: Not Impossible but Not Bloody Likely