All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is reported in a sound Hadith from Umar Ibn Shuaib reporting from his father who, in turn, reported from his grand father that the 'Prophet denied getting into circles (for knowledge or admonition) on Friday before Friday prayer and he also denied buying and selling in the mosque.'
The author of Awn Al Maboud explained this by stating that the Prophet denied getting into circles or groups for knowledge or admonition.
Al Khattabi said that the person is ordered to occupy himself or herself in the prayer, listening to the Khutba and the Dhikr. After finishing prayer, people can get into groups or circles to acquire knowledge or do similar good activities. Al-Tahawi said: ‘It is dislikable to get all the time into knowledge circles or groups before Friday prayer. But if this is done from time to time, it is then acceptable’.
Some scholars think that getting into knowledge circles on Fridays is dislikable in itself while other scholars believe that it is dislikable because it delays the lining up in rows which is required.
Anyway, it is more appropriate that the admonition or lecture you talked about be given after the prayer in order to let people concentrate on Dhikr, Qur'an and Prayer.
Allah knows best.