While at Corpus Christi, in the period 1653–54, Rosewell was prescribed for "a very hot stomach" by the apothecary, Jeremiah Webbe. From 1666, he made regular contributions to the "New Building", Corpus Christi College, reaching a total of £744 by February 1668.
Reputation
Rosewell succeeded Thomas Mountague as Head Master of Eton in 1671 and was the last "alien" to be elected to the position for many years. He was held in high esteem for his learning. Rosewell seems to have greatly increased the reputation of the school, and is styled by one of his pupils, ludimagister celeberrimus. A manuscript records in 1678 that "then it was that the foundation of its present grandeur was laid".
During his period as Head Master, in 1678, a list of the school was produced — the earliest complete list extant — giving the names of 207 boys, 78 of whom were in College. No other list is forthcoming for another forty years. It is written, not printed, on a half-sheet of parchment. The seventh form had long been abolished, the sixth being the highest. In each form, except the sixth, the names of Collegers and Oppidans are given separately. In this list also, brothers were distinguished by the style of major, minor, and minimus, and so forth, as at the present day.
Resignation
In 1678, Rosewell obtained a canonry at Windsor, and in 1682 resigned the headmastership. According to a rumour of the day, his resignation was caused by his falling into a fit of melancholy madness, in consequence of having killed a boy by immoderate flogging, and fancying that the King's messengers were coming to arrest him. The story does not sound very probable, and the less so as he was elected a Fellow of Eton in 1683. His successor as Head Master was Charles Roderick, Etonian and Kingsman, who had been Usher from 1676.
Will
John Rosewell wrote his will on 24 June 1684 leaving his valuable library to Corpus Christi College, Oxford; £300 for "the benefit and use of Eton College School"; £200 to the Church of Windsor; £50 to his "kinsman" William Rosewell ; £40 to the town of Berwick-upon-Tweed; and £20 towards the erecting and building of a Chapel for St Mary Magdalen Hall, Oxford. He died on 30 October 1684.