Former D.C. school superintendent Vincent E. Reed, now an assistant U.S. Secretary of Education, and an unprecedented grouping of 12 superintendents from the Washington suburbs yesterday strongly denounced the education tax credit initiative on the city's ballot Tuesday.

Reed's statement that he is "violently opposed" to the $1,200 per pupil credit from D.C. income taxes came despite support from the Reagan administration for federal tax credits to aid private schools.

Earlier, Reed had generally endorsed statements by President Reagan and Education Secretary Terrel H. Bell, favoring the federal credits, which, under bills now before Congress, would start at $250. But he had kept silent on the local measure despite urgings from its foes that he speak out against it.

"I'm opposed to the tax credit in D.C.," Reed said yesterday, "because I think it's too high and I think people have not been given proper information." But he added: "I don't disagree with President Reagan or Secretary Bell at all. I disagree with the tax credit proposal for Washington, D.C., that's on the ballot next Tuesday . . . It's a matter of that particular issue. I have no problems with some form of tax credit . . . "

The tax credit proposal in Washington could be used for students at either private or public schools and be taken by any person or corporation that pays taxes in the District of Columbia. The proposal has been supported principally by a local arm of the National Taxpayers Union. It has drawn vigorous opposition from virtually all city and school officials.

At a separate press conference yesterday morning at D.C. school headquarters, the suburban superintendents described the Washington ballot measure as part of a widespread attack on public education. They said that if the proposal were approved in the city, it might spread later to their own communities.

"This is the first time in history that this group of public school superintendents have gotten together publicly on any issue," said Montgomery Superintendent Edward Andrews, chairman of the area-wide superintendent's group.

"If the tax credits pass here, it would set a dangerous precedent that could have disastrous consequences for the support of all public schools," Andrews continued. "It might spread, and we see that as a possibility."

Bill Keyes, chairman of the D.C. Committee for Improved Education, which collected over 27,000 signatures to put the proposal on the ballot, said the superintendents joined to attack the measure because they are "afraid that if parents had the ability to control their public school systems and opt out of them, then many people would do that in their own systems as well as in Washington.

"Obviously, they all want to keep the control they have over over the monopoly system in their areas," Keyes said.

Commenting on Reed's statement, Kent Guida, a spokesman for the tax credit committee, said, "Perhaps there is a political reason why he thinks it's not a good thing for him or the Reagan administration to be associated with . . . I don't know how he would differentiate between a D.C. and federal tax credit except by the dollar amounts involved."

The phalanx of suburban superintendents who opposed the tax credits span a wide range of educational philosophies. Nine of them made a joint appearance with D.C. superintendent Floretta McKenzie, who has fought vigorously against the tax credit initiative since she succeeded Reed in July.

Twelve superintendents, along with McKenzie, signed the joint statement released at the press conference, urging D.C. voters to reject the proposal.

The superintendents said that McKenzie had not asked for their support.

"We invited ourselves," said Fairfax Superintendent Linton Deck. "It's an issue that transcends the boundaries of the District of Columbia in a very obvious way." Besides Deck and Andrews, those at the press conference included Charles Nunley of Arlington; Robert W. Peebles of Alexandria; and superintendents from Fairfax City, Falls Church, Loudoun, Prince William and Charles counties. The statement also was signed by Prince George's Superintendent Edward J. Feeney and Howard County Superintendent M. Thomas Goedeke, both of whom sent top deputies to the press conference, and by Manassas Superintendent F. Brent Sandidge.

Deck said a suggestion by tax credit supporters that the money might be used to help black children from the District of Columbia pay tuition at suburban public schools is "not realistic."