Grade: B-

If ever there was a critic-proof movie, this is it. After two cable flicks that literally amounted to a cultural phenomenon, “High School Musical 3: Senior Year” has a huge built-in fan base that will not only turn out the first weekend but revisit the movie regularly and snap up the DVD when it finally comes out in that medium.

That said, one must regretfully report that the third installment is the weakest of the bunch. Partially that’s because of the extreme thinness of the plot that Peter Barsocchini has confected this time around, even by the standards of the first two pictures, which were hardly Shakespearean. Though things begins with Troy (Zac Efron) leading his team to a come-from-behind victory in the state basketball championship game, the focus is on him and Gabriella (Vanessa Hudgens) struggling to deal with the separation that will come with graduation (or, as it happens, before that) and the difficult decisions they face about where to go from here. But despite all the hard choices confronting them, they take the time to star in the senior musical being written by Kelsi (Olesya Rulin), choreographed by Ryan (Lucas Grabeel) and manipulated for her own benefit by Sharpay (Ashley Tisdale), as well as preparing to go to the prom together. And to do lots of extracurricular singing and dancing, in which their respective pals Chad (Corbin Bleu) and Taylor (Monique Coleman) play major parts, too.

Simply put, this scenario doesn’t even match the depth of the first two pictures, which were shallow fluff to begin with, especially because torturous contortions are needed to create excuses to keep the lovebirds apart. And the ending, in which all the dilemmas and difficulties are resolved like magic without anybody breaking a sweat, is a cop-out even by HSM standards.

But that’s to be expected. A more severe problem with the movie is that the songs are, at least at first hearing, extremely weak both in terms of the bland tunes and the repetitive lyrics—nowhere near the catchy quality of those in the first movie. Increased familiarity might change that—but I doubt it.

There’s considerable compensation, though, in the dancing, which is on a much more extravagant scale than before. Director-choreographer Kenny Ortega has gone for broke here, and the result is pretty spectacular in several cases, especially in the “Boys Are Back” number than Efron and Bleu perform together at roughly the mid-way point. They may not be Kelly and O’Connor, but it’s the closest the movie gets to the old standards, and quite enjoyable.

The other major strength of the picture is Efron, who continues to show real star quality—a sensitivity that will appeal to adoring female tweens without turning off guys in the process. He’s also got the athleticism that allows him to appear both authentic on the basketball court and to pull off a strenuous solo dance routine in a whirling corridor (a bit of homage to Fred Astaire), though the song he’s singing at the time is as mediocre as the rest. Hudgens is an acceptable partner for him, but is distinctly pushed into the background in this case, while Bleu and Evans continue to show sass and Tisdale is plausibly pushy. Unfortunately, the new kids on the block—Jemma McKenzie-Brown as an English transplant who tries an “All About Eve” routine on Sharpay and Matt Prokop as a doofus teammate of Troy’s—do not seem to be likely lead material for any further sequels. The adults, needless to say, are pretty much beside the point, except for Alyson Reed’s drama teacher Ms. Darbus, who’s as much a caricature as ever. Visually the movie maintains the vibrant candy-colored look of its predecessors, with Mark Hofeling’s production design, Wing Lee’s art direction and Caroline B. Marx’s costumes all contributing to the modern fairy-tale appearance, and Don Brochu’s editing lets us see a lot more of the dance numbers from a distance than is often the case nowadays.

Finally, it may be noted that there’s a distinct plus to the HSM franchise in the simple fact that whatever its flaws, the series has served to awaken an interest in live musical theatre among its young fans. Professional staged versions of the original have been a success, and it’s performed in high schools across the country, too. And Broadway has just seen the opening of a new show called “13,” aimed at youngsters of the HSM base. So if “High School Musical 3” isn’t Rodgers & Hammerstein or Lerner & Loewe, the fact that it and its predecessors have served to introduce a new generation to the genre would be reason enough to welcome it.

And whatever the flaws, it’s lively and colorful enough to satisfy the target audience as a fitting close to the ultra-popular series.