The N.F.L. is the last major professional league to refuse to draft underclassmen or players with eligibility remaining. The National Collegiate Athletic Association has said it cannot prevent these players from leaving. But Richard Schultz, the N.C.A.A. executive director, recommended last December that players who leave school for the draft be permitted to retain their eligiblity.

The American Football Coaches Association has hinted that it could prevent pro scouts from visiting campuses if the league altered its policies. This refusal would make it extremely difficult to gauge a player's health or speed since testing would be closed to the scouts.

Until now, the N.F.L. has permitted only players whose class had graduated or who had used four years of eligibility to enter the draft. In certain circumstances players were allowed to enter a supplemental draft when they had graduated with their classes but had not used up their eligibility. Last year, the league allowed Barry Sanders, the Heisman Trophy winner from Oklahoma State, to enter the regular draft even though he had eligibility left and had not graduated. The reason given was that his school was going on probation, although the threat of a legal challenge to the N.F.L.'s policy was also thought to have had a bearing on the decision.

Wanted Consistent Policy

Tagliabue has said that the league needs a consistent policy, and once it draws the line on juniors, it should not deviate.

By deciding now to permit them to enter the draft, the N.F.L. will give its scouts the opportunity to examine and test the players before the draft on April 22. Only seniors were tested at the annual collegiate ''meat market'' extravaganzas in Mobile, Ala., and Indianapolis.