The correct sentence is: "Everyone brought their dictionary for reference in the class."
The error here is that "everyone" is singular, but in this context, it's treated as a group, and it is common to use "their" to refer to people in a general sense, even though "everyone" is singular.
So, "their" is used for inclusivity, but grammatically, "his or her" could also be used if you prefer to maintain strict singular agreement. However, "their" is the more commonly accepted usage in modern English.
