What is the ruling on students placing a finger in another student's anus over their clothes, and what is the ruling on one person lightly touching another student's buttocks in the queue, and is this act considered sodomy?

What the students are doing is a great evil (munkar), and you must warn and advise them. Their guardians must reprimand and discipline them. As for what you felt, a touch from behind you during the line-up, the perpetrator must be stopped. There is no sin or blame upon you for this, and this is not an act of sodomy (liwat). The sin is upon the one who did it.