In the name of Allah, the most Beneficent, the most Merciful.  

Answer

In the aforementioned scenario the marriage will be done.  Saaiduna Abu Hurairah Radiallahu Anhu narrates that the Prophet of Allah Sallallahu Alahi Wasalam said “Three things – their seriousness is serious and their jesting is serious: Marriage, Divorce and Rajaat (revoking the divorce to the wife)”.   (Mishkaatul Masaabeeh p.284)

There are two conditions for an Islamic marriage to be valid.  Firstly, the man or his representative or the woman or her representative or her custodian (i.e. her father, brother, uncle etc.) say the statement of offer and acceptance. These statements should be said in the past tense, or one of them expressing the past and the other the future, such as one saying, “Marry (your daughter) to me” and the other saying, “I have married (her) to you”.   Secondly, the physical presence of two free, adult, sane, Muslim male witnesses, or one man and two women, whether they be morally upright (adil) or non-upright. (Mukhtasar Quduuri p.140)

In the situation you mentioned above, as the boy and girl uttered statements which indicated offer and acceptance and two witnesses were present, the marriage will be done despite having been done so in jest.

Only Allah Knows Best

Mohammed Tosir Miah

Darul Ifta Birmingham