There are many rituals and conversational punctuators which seem specific to the school playground. Two of note are 'belming' and 'chinning'.
Belming consists of inserting the tongue between the lower lip and the teeth, producing a comedic swelling of the lower jaw area. It is usually used to indicate that the person you're speaking to has just said something really, really stupid. For instance, your friend may note that the physics teacher and the physical education teacher have the same surname. He may then say 'Is Mr Brown married to Mrs Brown?'. You, of course, have information he does not, allowing you to ridicule him thus: 'No, because Mrs Brown's a lesbian, innit? (Belm)' or similar, where 'Belm' indicates that you have just performed the tongue-behind-lip movement1.
In extreme cases the belmer may actually say 'Belm' as they do it, producing a very specific kind of sound. This is usually used for examples of particularly spectacular ignorance or stupidity. For example, your hypothetical mate mentioned above may come back with 'Is she? Which part of Lesby does she come from?' In response to something like that, you may struggle to get enough of your tongue into the gap between your teeth and lips.
The origins of chinning are a mystery. It exists in a number of forms throughout the UK, varying in the exact details of motion and the words used. In all cases, though, scepticism of something believed to be an outrageous lie is indicated by chin-stroking.
The above description does not really do justice to what can be a really exaggerated movement. A 'chinning' may vary from a brief, light pinch of the chin with the first two fingers and thumb, while raising eyebrows, all the way up to miming the stroking of a grotesquely huge chin stretching out horizontally to arms length from the jaw while shouting 'Tutenkhamoooon!'. One might employ the former in response to the statement 'My dad's harder than your dad. He's got a black belt in karate.' One might employ the latter in response to the statement 'My dad's harder than your dad. He's world champion at fighting.'
The archetype, however, is the full, four-fingered chin stroke performed while thrusting the lower jaw forward, accompanied by the words 'Chinny reckon'. There is an important point of pronunciation here. 'reckon' is normally pronounced to rhyme with 'beckon', with the emphasis on the first syllable. However, when chinning, it is vital that the word is pronounced with the emphasis heavily on the second syllable - 'Chinny reck-ON!'
Various theories of the origin of this practice have been put forward, some of them referring to the sports broadcaster Jimmy Hill, but none can be proven conclusively.