IS IT A SIN to enjoy frothy musical fluff? If so, the line for confession starts here. The Olney Theater has mounted an encore of John Powers' good-natured "Do Patent Leather Shoes Really Reflect Up?" and though the show is as inconsequential as they come, Olney's chipper cast is devoted to making it as much fun as possible, and does so, overcoming some formidable obstacles.

Powers' skit-like script -- it could be called "The Official Catholic Handbook" if the Baltimore Catechism didn't own that title -- manages to fit in the whole litany of Catholic school trademarks: saints' days, rosaries, impure thoughts, your Permanent Record. It may be that enjoyment depends largely on familiarity with pre-Vatican II Catholic conditions.

Powers, who adapted the script from his amusing little book of the same name, is no powerhouse in the dialogue department. He owes the good sisters all the credit for his few memorable (and realistic-sounding) lines: "Make room in your seat for your guardian angel," and "If you want to go to heaven, you must learn to suffer."

Unlike Christopher Durang and other playwrights who have recently aimed a searchlight on Catholic schooling, Powers doesn't seem to have an axe to grind here. His is an affectionate ribbing; in his sentimental memory, everyone turned out all right in the end.

The first act, tracing the elementary school years of a standard gang of Catholic kids, is the more engaging of the two, though it's hard to stomach adults pretending it's their first day of school. Act two, the high school years, slides into the "Grease" pit of stereotypically dippy '50s scenes -- the sock hop, make-out crises, etc. The score, by James Quinn and Alaric Jans, is forgettable pop pastiche, with rhyming-dictionary lyrics.

While all that doesn't sound too promising, director Bill Graham Jr. keeps the show moving at a zippy pace; at one point he trots out a trio of singing nuns who give new meaning to the Andrews Sisters sound. The cast, largely composed of Catholic University drama students, is lively and appealing, and though the acting is a mite sugary and sunny for some tastes, there are quite a few fine singers, most notably Maisie Mountcastle as Sister Helen. Music director Rob Bowman gives the thin music a professional polish, and provides some heavenly vocal arrangements.

So, though "Patent Leather" may not take you as far as show-biz heaven (limbo, maybe), you certainly won't suffer much on the way.