“I’ve got to fill out,’’ O’Gara, a 6-foot-3-inch, 185-pound defenseman, said with a smile yesterday.

If Seidenberg (6-1, 210 pounds) is a hydrant, O’Gara is the pole firefighters traverse en route to their trucks. He has the spindly arms and thin torso that underscore the fact that he only turned 18 last Wednesday.

“I’m definitely behind physically,’’ said O’Gara, the 151st overall pick of the 2011 draft. “I’ve got to get with the pace on the ice. It takes an adjustment. It’s going to take a lot of work.’’

Twenty-four youngsters have rolled through the Bruins’ development camp, which concludes today. There are the high-end prospects such as Dougie Hamilton, the team’s most recent No. 1 draft pick, who have taken the traditional route. Hamilton has played major junior hockey for the last two seasons for Niagara of the Ontario Hockey League. He has participated in Team Canada’s Under-18 and U-17 camps. Next month, Hamilton will participate in his first development camp for Canada’s entry for the 2011 World Junior Championship.

O’Gara, meanwhile, is one of a shrinking crowd: the prep school star.

It was just a decade ago that puck prodigies such as Brian Boyle, Kevin Regan, Sean Sullivan, Mike Morris, and Kenny Roche swaggered through their high school years at St. Sebastian’s. Now, such players are opting for the junior ranks of the USHL or EJHL. Or, in the case of Americans such as Jared Knight (Battle Creek, Mich.), one of the Bruins’ 2010 second-round picks, they’re jumping to Canadian major junior.

Not O’Gara.

Had he remained home in Massapequa, N.Y., he would have graduated from high school with his friends last month. But last summer, O’Gara was recruited by Milton Academy coach Paul Cannata. After a visit to Milton with his parents, O’Gara decided to repeat his junior year and enroll at the ISL school. Hockey wasn’t even the first consideration.

“When I chose Milton, with the NHL, I couldn’t have even told you it was my draft year,’’ said O’Gara, who will enroll at Yale in 2012. “It’s just been a whirlwind since. Getting to school was my biggest priority.’’

Once O’Gara, a left-shot defenseman, kicked off his Milton career, he grabbed scouts’ attention. The Bruins saw plenty of O’Gara. They liked his size and his smarts. Even if he was a wild card because of the route he’s chosen.

It’s easier for scouts to project a teenager when he’s playing juniors - CHL, USHL, or EJHL. These days, it’s where most of the draft-eligible players are.

Asked whether it is harder to project high school players, assistant general manager Don Sweeney, who starred at St. Paul’s, said, “Probably a little bit. You have to have a feel for the league, have guys that have come out of there, and how their trajectory has gone. It’s a harder route. I went that path. It’s definitely a harder route. You don’t play as many games.’’

For O’Gara, the development camp has offered more than for the other junior players. For the first time, he has experienced training, on and off the ice, under professional people and alongside older players with pro experience. In the dressing room, O’Gara can look to his right and see 22-year-old Tommy Cross, a defenseman with a similar history.

Cross played high school hockey at Westminster School in Simsbury, Conn., before being picked in the second round of the 2007 draft. Cross will be a senior captain at Boston College this fall.

In yesterday’s scrimmage, Cross, while paired with Hamilton, was among the smoother two-way defensemen. Cross is the same height as O’Gara but is 25 pounds heavier. O’Gara still looks like a boy; Cross is a man.

“He’s huge,’’ O’Gara said. “I’ve got to fill out. Just his patience. He’s so smart on the ice. He’s always in the right spot. Watching a guy like that can really help me grow in my game.’’

O’Gara skated alongside Marshfield native David Warsofsky yesterday. Naturally, O’Gara didn’t look as smooth or poised as Cross. But teams can afford to take some gambles in lower rounds. O’Gara already has a good stick. The Bruins hope that once he packs bulk onto his frame, he will become more powerful. He’s already got the height that pro teams like.

“Things are kind of new to him,’’ Sweeney said. “You can tell he’s a little wide-eyed and trying to get himself up to pace. But that’s a good thing from where we sit. He’s got the next four, five years, whatever he needs, to develop into the player we think he can become. He’s not getting any smaller. Hopefully he only continues to get bigger.

“He could be like Tommy Cross, coming back for four or five of these camps. You can see him continue to grow and develop as a player before we even think about him turning pro. That will map itself out. There’s no course of action laid out for each and every one of them. They’ll set their own course.’’

Fluto Shinzawa can be reached at fshinzawa@globe.com. Follow him on Twitter @GlobeFluto.