I am not a teacher but a student. Anyway, your post aroused my interest and I started searching on Google examples of both expressions.
By the looks of the first results I was tempted to say that "in hysterics" is more like saying "very anxious", "nervous", "scared"; or even "in laughter", whereas "hysterical" could involve sudden physical reactions, specially shouting and/or crying.
I mean, it suggests (to me anyway) a state of quietness and stillness (well, except for the hysterical laughter that happens when someone is in hysterics).
These are quotes from the websites I've found.
But then I went on further to find that sometimes "in hysterics" is used to refer shouting and crying too:An hysterical shriek: "OH! GOAL! IT'S A GOAL!"
The Tolkien community is in hysterics today as we finally get the ‘official’ announcement concerning ‘The Hobbit.’
Paris tells People she was ‘in hysterics’in jail
LOS ANGELES - Paris Hilton says she was “in the fetal position, basically in hysterics” during her first few days in jail.
One must have the naïveté of a child -- which cannot be said of my most reverend opponent -- or to be in a state of hysterics, to pretend to remember exactly some conversation which was supposed to have taken place fourteen years ago.
Shoppers in Hysterics Over TMX Elmo
(Elmo) explodes into fits of hysterical laughter when he is tickled.
Mamet's Court Is In Hysterics (theater review of Romance by JEREMY McCARTER)
What the hell has gotten into David Mamet? The puzzlement is genuine, and the profanity is the cooing of a dove compared to the bad words Mr. Mamet unleashes in "Romance," his outrageous new comedy at the Atlantic.
I’ve been sent into hysterical laughter everyday this week. And I have a laugh which is not to be messed with. My laugh is so loud it will find you 100 feet down the hall around the corridor and into your work station. Especially when I’m in hysterics. I try so hard to keep it quiet, but I can’t. This is even harder when I’m trying to tell a funny story to someone else, and to me the story is so funny that I can’t tell it.
I'm basically bouncing this post since I can't conclude a rule from the above. I have got an idea, but I would like to know the NES point of view.(Baby) in hysterics after he saw a remote controlled car!
The partner of drowned grandfather David Ball has been rushed to hospital in "hysterics" after he died trying to save her during a dream holiday in Australia.
Linda, 59, made it to the shore but the Evening Post was told she has now been admitted to hospital in the Brisbane area after becoming hysterical following the tragedy.
I'm sure some teacher will tell us something pretty soon.