Secondary school headteachers have written an open letter to voice their concerns over students' behaviour, which includes punching holes in walls, cutting computer cables and smashing ceiling tiles.
The letter, from some headteachers in Carmarthenshire, says the impact of the pandemic is becoming very clear. It includes the signatures of ten headteachers next to their school emblems.
Addressed to parents, the letter points out widespread "detrimental" behaviour, which could even lead to "criminal investigations and prosecution."
It raises particular concerns over "inappropriate" graffiti on equipment, with the intent of abusing schoolmates and staff.
The headteachers also highlighted the "significant" time pupils spend on social media or on gaming websites, some of whom were said to be doing so in the early hours of the morning.
They also said they worried about the abusive message that can be posted on social media apps.
"There is a marked increase in the abusive and threatening nature of comments posted in group chats, with individuals hiding behind the fact that comments either disappear after a period of time, or other chats allow anonymous comments to be posted," the heads added.
Within the letter they also urged parents to take screenshots of abusive posts and report them to social media platforms. The heads said online harassment can be reported to police and culprits "could face criminal prosecution".
The schools also said they are having to invest a "significant amount of staff time" to prevent pupils using electronic cigarettes during the school day.
In response to the letter, a Welsh Government spokesperson said: “We’ve more than doubled mental health support for schools, to over £12million, since the start of the pandemic as part of our ‘whole school’ approach to mental health and wellbeing.
"Counselling services currently see around 11,500 learners each year and support for staff will be trebled this year.”