You're right, it is confusing! I've had to look at different sources to piece it together. What happened was that Michael left Eton at the end of the summer, intending to enjoy a last summer holiday before joining the Scots Guards in the autumn. However, he changed his mind and went back to Eton for the Michaelmas Half while waiting to join the guards. Hugh Macnaghten wrote that "subsequently he [Michael] wrote in the quiet of the holidays asking if he might return. He was welcomed back, and the record of his last Half is 'A wonderful Captain: he worries, but his judgment is unerring and his actions swift as lightning: the most admirable boy who has ever been in the house.'
The war ended in November, and Michael was finally persuaded to go to Oxford in January 1919.
Hope that helps!