The laws about knives are quite complicated.
Across the UK and Republic of Ireland, it is against the law to carry a knife or an offensive weapon in a public place unless you have a good reason.
If police think you are carrying an offensive weapon they can stop and search you.
Smaller knives - like pen knives - can confuse things a bit as they are not illegal as long as the blade is shorter than 7.62 cm (three inches). But it all depends why you're carrying it.
Teachers can search pupils for knives
It's OK if you're going fishing and have a penknife in your bag to use while you're fishing.
BUT if you're carrying it for protection or to use as a weapon, then that's against the law.
It's very unusual for someone under the age of 16 to have a good reason for carrying any kind of knife in a public place, so it's safer not to do it.
This doesn't always stop children from taking knives to school to "defend" themselves or using them to threaten others.
To help stop this, in 2007 teachers were also given powers to search children for knives at school.
Metal detectors have also been introduced at some schools to prevent kids taking knives and other weapons into class.