It's a reduced relative clause - a shorter, neater version of this: "This is just a group of people [who are ] trying to steal something from school.

It's not a gerund in this situation it is present continuous/progressive tense. I am stumped why we in my country don't use the (are) part in this case. I know it's because of (This is) but I cant tell you why grammatically. We say the following People are trying to steal something from school A group of people are trying to steal something from school. but when we say This is just a group of people are trying to steal something from school. it sounds wrong. And I can't tell you why exactly. We would say This is just a group of people who are trying to steal something from school. There is something about the phrase 'This is just a group of people' that changes the construction and it has something to do with using 'this is' being singular mixed with a plural subject. Lets hope some clever person comes along that can enlighten both of us

It would also be correct if you included the verb 'to be' after adding a relative pronoun: 'This is just a group of people who/that are trying to...'.