When I was in primary school, my teacher discouraged my classmates from writing with their left hands, and even forced left-handed children to use their right hands. Every time they used their left hands, they got a whack from the teacher.
Currently, I've got some foreign students who are left-handed. One of them used to do the voluntary work in Tibet. She told me that one Tibetan person, who saw my student writing with her left hand, got angry and took the pen from my student's heft hand and put it into her right hand.
There is no reason explaining that writing with your left hand is bad, but in some Vietnamese's mind, being left-handed is kind of abnormal and it should be prohibited. In Vietnam, people make their chilrden do everything with their right hands: writing, using chopsticks, ...
In Vietnamese culture, individuals were programmed to follow the mainstream. In most Asian countries, the left-handed people were believed having something wrong in their minds. With Tibetan, left hand is dirty and is not used in writing, eating, ... In the past, Vietnamese people were afraid that their children might be disadvantaged when they used their left hands to write.
Nowadays, with many scientific evidents, I think all of you know the answer. Left-handed people are mostly talented, good at organizing and smart.
The Ministry of Education and Training in Vietnam has never promulgated any regulation prohibiting students from using their left hands for writing. Therefore, I hope that no teacher forces their students to write with their right hands.