Most of that is centered around Karen Gillan's costume, which shows a great deal of skin in comparison to the rest of her male co-stars. She recently posted a response to fans saying that it would all make sense in time.
Jumanji! Yes I'm wearing child sized clothes and YES there is a reason! The pay off is worth it, I promise! #Jumanji
A recent tweet from Mashable reporter Jeff Sneider supposedly sheds some light on the plot, and it does seem to make sense.
JUMANJI sequel said to follow 4 teens in detention who get sucked into the game, where they take on new "avatar" forms to locate a jewel.— Jeff Sneider (@TheInSneider) September 20, 2016
"JUMANJI sequel said to follow 4 teens in detention who get sucked into the game, where they take on new "avatar" forms to locate a jewel."
Later in the comments, a few other nuggets of information managed to surface, including the possibility that Jack Black's teenage character is actually a girl who picks the form of Jack Black as her Avatar. Also of note is this snippet from Chris Pugh, who says "I think it's worth noting that it's a video game console they find stored in the school, & a different version of Jumanji."
Granted, at this point none of these facts are confirmable, and they likely won't be until a bit later down the line, as Jumanji doesn't release until mid-2017. Still, going the game console route would be an easy way to modernize the premise, and having the teens pick their own avatars would provide some great opportunities for character development if the script can deftly handle them that is.
Jumanji hits theaters on July 28th, 2017.