Lennox said it led to some tense moments but Collegiate kept Rathkeale in their own half till the fulltime whistle.
"They rammed everything at us but we held on."
It was the team's first win in their third Central North Island First XV competition game and is the start of a busy period that will test the squad's depth.
They have games every Saturday and Wednesday until July 6 a mixture of Central and inter-school fixtures.
"Six games in an 18-day period, it's all on for sure," said Lennox.
He said a number of younger players stepped up on Wednesday so there is good competition in the squad now.
Collegiate play away to Feilding High School on Saturday.
Wanganui High School First XV will contest the second round of the Manawatu Colts competition at home against Te Kawau on Saturday, a return game from two weeks ago.
The side lost 30-13 to Old Boys Marist RFC last week, although coach Darryl Malcolm said they enjoyed playing at FMG Stadium during Marist's club day as a curtainraiser to the premier teams.
Vilimoni Koroi scored their try while Sam Malcolm landed a conversion and penalty.