WASHINGTON (CNN) -- With his chief political aide under investigation as part of a probe into the public unmasking of a CIA operative, President Bush is sending his staff back to school -- ethics school.

Bush is requiring his executive office staff to attend refresher courses on ethics and handling classified materials, according to a White House memo.

"The President has made clear his expectation that each member of his Executive Office of the President (EOP) staff adhere to the spirit as well as the letter of all rules governing ethical conduct for EOP staff," states the memo sent to Bush's staff.

Staff members with security clearances will attend mandatory sessions next week, and those without security clearances will attend mandatory sessions the following week.

The memo went to all EOP staff, which numbers about 3,000, White House spokesman Scott McClellan said.

The refresher course comes as Bush's top aide, Karl Rove, is under investigation and as Vice President Dick Cheney's former chief of staff, I. Lewis "Scooter" Libby, faces indictments in connection with the outing of a CIA operative.

Libby, who resigned October 28, pleaded not guilty Thursday to charges of perjury, obstruction of justice and making false statements.

Libby is accused of lying to a grand jury and FBI agents about where he first learned Valerie Plame's identity and what he later told reporters about her.

Plame is the wife of former diplomat Joseph Wilson, who had openly criticized the Bush administration.

Libby is not charged with deliberately disclosing the name of a covert agent, which is a federal offense.

Bush has declined to talk publicly about the investigation. On Friday, he deflected questions about Libby and Rove at the Summit of the Americas in Argentina.