Nonsense I'm afraid. Cambridge was certainly bombed http://www.cambridge-news.co.uk/pictures/nOST-ALGIA/pictures-26199628-detail/pictures.html http://news.bbc.co.uk/local/cambridgeshire/low/people_and_places/history/newsid_8754000/8754009.stm Oxford was, perhaps difficult for the German bombers to identify? It was certainly a legitimate target as the Morris motor works were there at Cowley, building military vehicles, amongst others. Other cities without particular military importance were deliberately hit in what were referred to at the time as Baedeker Raids http://www.historylearningsite.co.uk/world-war-two/world-war-two-in-western-europe/britains-home-front-in-world-war-two/the-baedeker-raids-of-1942/

No, such an agreement would have seemed ridiculous. No one felt the Germans could be trusted to honor any agreements, and rightly so. It did turn out that Hitler planned to relocate the capital of the United Kingdom to Oxford, but that is not why the university cities were spared heavy bombing.

The reason university cities on either side were spared was simple military necessity. You don't risk lives and materiel to conduct a bombing raid on a target with zero strategic value. You focus on oil production facilities, factories, naval facilities, rail transport lines, command and control centers, and other targets that will damage the enemy's capacity for making war. Strategic bombing was not wholly indiscriminate. Even Dresden was selected so as to create panic and chaos in Germany to aid the Soviet advance.