A Clue in Code by Marilyn Singer, 1985.

This book is part of a series about a pair of twin boys, Sam and Dave Bean, who solve mysteries with their friends.

One day, the boys’ teacher, Ms. Corfein, has one of their classmates, Roger, collect money from the students for a class field trip.  Ms. Corfien tells Roger to put the money in her locker, but later, the money is gone.

Anyone in the class could have taken it.  When Dave went into the classroom to feed the class’s pet gerbil at lunchtime, he saw Willie, the class bully, there as well as his friend Patti, who thought that it was her turn to feed the gerbil.  Of the two, Willie seems like the best suspect for the theft because he’s been in trouble before for stealing, but perhaps there was someone else in the room before any of them arrived.

Sam and Dave decide to begin their investigation with Willie.  His father is the school’s custodian, and Willie has been using a copy of his father’s key to the school to sneak around after hours.  To the twins’ surprise, when they confront Willie, not only does Willie deny stealing the money, but it turns out that he’s actually been scared and upset himself.  After being caught stealing the last time, his parents have been especially strict with him, sending him to bed early, limiting his time with friends, and not allowing him to watch tv or read comic books.  In fact, his father tried to get rid of his entire collection of comic books by throwing them in the trash, but Willie rescued them and has been hiding them in the school, sneaking away to read them when he can.  However, someone has discovered them and stolen them.  Willie wants his comic books back, but he can’t report them stolen because his father thinks that they’re already gone.  Worse still, his parents will be even harder on him if he ends up taking the blame for stealing the class’s trip money.

In spite of Willie’s reputation, which he deserves, Sam and Dave think that he’s telling the truth about the thefts.  Then, they happen to find a strange message that appears to be written in code after some of their classmates were throwing paper airplanes.  With the help of Rita, a friend who is very good with codes, they decode the message and uncover a valuable clue that explains why the comic books were taken and leads them to the thief’s identity.

I was pretty sure, from the very first chapter, who had stolen the money, but I wasn’t completely sure of who had taken the comic books until the end.  At first, I thought perhaps Willie’s father had found his stash and got rid of it again, but that wasn’t the case.  The thief who took the comic books was the same person who took the trip money and for the same reason.  This person’s father has been out of work, and they need the money.  It turns out that Willie has a number of comic books that are now collectors’ items, and someone is willing to pay a lot of money for them.