Tehran Bureau provides some historical background on the 7th of December - "Iran's Student Day":
After the CIA/MI6 coup of August 18, 1953, when the popular government of Dr. Mohammad Mosaddegh was overthrown and Shah Mohammad Reza Pahlavi was put back on the throne, campuses became even more political. In the immediate aftermath of the coup, an extremely repressive and oppressive environment prevailed in Iran. The universities remained the most important places where protests against the anti-nationalist and foreign-sponsored coup were taking place.
On November 15, 1952, the coup government announced that Richard M. Nixon, then Vice President to U.S. President Dwight D. Eisenhower, would pay a visit to Iran on December 9, 1953, presumably to celebrate with the Shah the demise of the Mosaddegh government and restoration of the monarchy. Nixon's visit was also supposed to demonstrate the Shah's full support for the United States. At that time however, anti-American feelings were running very high in Iran. Despite the extreme repression, the Shah had not been able to completely crush the opposition. The news of Nixon's trip angered the frustrated population, especially the opposition. [...]
On the morning of December 7, 1953, the guards entered [the University of Tehran], the heart of the protests, to prevent any repeat demonstrations.
Though there had not been any demonstrations yet that day, the excuse given was that some students had mocked the police, and the police wanted to arrest them. Two soldiers and an officer went to a class to make the arrests. But the professor, Shams Malak Ara, asked them to leave. As they arrested two students, one student jumped on a desk and began shouting for help. Shams Malak Ara notified the Dean of the FOE.
The soldiers and the officer then went to the office of Dean of the FOE, Mohandes Khalili [who was later active in the National Front]. He also protested the intrusion, and his deputy, Dr. Rahim Abedi, was ordered to ring the bells to notify the students. Students gathered in the hall on the first floor of the school. The guards who had been on alert invaded the FOE building. According to Dr. Abedi, 68 bullets were fired. Three young students -- Mostafa Bozorgnia, Ahmad Ghandchi, Mehdi Shariatrazavi -- were killed.
Much more here.