aww the joys of muck up day ^^ - our school has./ (well was) only be opened for 4 years, and never really had much of a muck up day as such.. (since everyday was pretty much one- there was 12 people in year 12 and the school didn't really give a shit but anyways)
We did a few things, like water bomb the hallways... which we got into trouble for... due to our school being conjoined to a University/tertiary part- (same building and all) and students were slipping everywhere, we blew up hundreds of balloons (man that took a long time) and filled up the staff room so the teachers (all 4 of em) couldn't walk into it, or through it wihtout popping them all (the staff room was half the size of a normal classroom- the staffroom being the princapals office aswell)
we tied strings to all the doornobs and across a hallway so that teachers / students from the Tertiary part couldn't open doors.
We drew on the whiteboards with permanant marker (yes you can get it off witha white board marker but it takes forever) and we drew ALL over it/them.
We emptied bottles and bottles and bottles of flour onto the pavement (which is in the middle of the city, and pedestrians were staring at us all day)
and we sang loudly on the rooftop garden annoying all.
and yesh... thats a few ideas. (if you 'ave a rather small school that is)