This is the first result on google, so here's a script to freeze time.
local FreezeTime = readInteger("Bully.exe+7CEB48");
local Cancel = createForm(true );
local Repeat = createTimer(Cancel, true );
timer_setInterval(Repeat, 1000);
timer_onTimer(Repeat, function( )
writeInteger("Bully.exe+7CEB48", FreezeTime);
end );
I've only used it on the main clock, I doubt it would work on class or challenges.
When you want to freeze time, alt+tab out of the game, and run the script in cheat engine. When you want time to start up again, close the window which pops up when you run the script.
If you don't know how to use a script, open Cheat Engine, click the button flashing between red and green, choose the Bully.exe from the list, then on the tool bar click Table>Show Cheat Table Lua Script. Copy and paste the script into the box, then click execute once you want to freeze the clock.