President Reagan pledged yesterday to "keep right on marching" toward further arms agreements after next week's expected treaty signing, but he said the United States must not be lulled into a new period of detente allowing a secret Soviet military buildup.

Less than a week before his summit meeting with Soviet leader Mikhail Gorbachev, Reagan had harsh words for that period of broadly improved relations with the Soviet Union.

"More than a decade ago, there was a warming in U.S.-Soviet affairs that we called `detente.' But while talking friendship, the Soviets worked even faster on the largest military buildup in world history. They stepped up their aggression around the world. They became more repressive at home. We do not want mere words. This time we're after true peace," Reagan said.

"In the excitement of the summit, the treaty signing and all the rest, we must not forget that peace means more than arms reduction," he said.

In a speech to high school seniors and their parents in Jacksonville Veterans' Memorial Coliseum, Reagan said he and Gorbachev will "have words about Soviet expansionism" during their three days of meetings in Washington.

And he told one of the students during a question-and-answer session later that in his talks with Gorbachev he might find himself "bending his ear" on what Reagan said was a need for religious freedom and other reforms in the Soviet Union.

One student also asked Reagan to defend his "Star Wars" space-based missiledefense plan, prompting the President to compareit to "a gigantic gas mask."

Reagan recalled that gas masks were retainedafter poison gas was outlawed. The StrategicDefense Initiative, he said, was "a gigantic gasmask and maybe...the thing that could bring aboutthe end of nuclear missiles."

The President made no reference in his speechto Gorbachev's hour-long NBC television interviewon Monday night. Asked what he thought of theSoviet leader's presentation, Reagan said, "I havehad a respect for him ever since I met him."

When one student asked if Reagan was worriedthat Gorbachev's apparent popularity in the Westwould make the American people more receptive tocommunism, the president replied, "I have morefaith in the American people than that."

As for his own feelings, Reagan said, "I don'tresent his popularity or anything else." Thepresident, referring to his days as an actor,joked, "Good Lord, I co-starred with Errol Flynnonce."

When another student asked the president whatadvice he would like to give to Gorbachev, Reaganreplied, "To really stick to his program ofglasnost," or more openness in society, and "tomake their country like ours--a place that peopledon't want to leave."

Of particular importance, he said, would be"when the day comes that the people of the SovietUnion can worship God in the way they want to."

"Yes, I may find myself bending his ear on thatvery subject," Reagan said.

The summit in Washington, scheduled for Dec8-10, will be the third between the two leaders.They met in Geneva in 1985 and in Reykjavik,Iceland, in 1986, but failed to come to terms onarms control.

"For many years, critics around the world haveinsisted that it would be impossible to get anagreement along the lines we've now worked out,"Reagan said. "Six years ago, when I proposed theelimination of an entire category of U.S. andSoviet intermediaterange missiles, they sneeredand said I couldn't be serious."

Now, he said, "we are about to sign anagreement that will do just what I proposed."

"After the summit," Reagan said, "we will keepour negotiators working on an agreement that couldlead to cutting the U.S. and Soviet long-rangenuclear arsenals in half and reducing thedisparities in conventional forces--that is, thearmies that face each other in Europe.