Children are taught the correct formation of letters as they learn each grapheme (the written units) through their phonics teaching. It is really important that children hold their pencil with the correct grip and teachers will always encourage this. Occasionally, children really struggle and develop their own style but this is less likely if it is picked up upon in the early years.
Children are shown how to join letters as they begin to learn about phonemes (the sounds) with more than one grapheme in. As the children progress through the years, they follow a scheme called Nelson Handwriting which develops more and more joins. Click on the link to see a short clip of the four main handwriting joins that are taught in school.
By the time children reach UKS2, most children will be able to write fluently in joined up handwriting.