LATHROP — A pent-up demand for charter schools means the South County’s newest public elementary campus - River Islands Technology Academy - will open in August with a waiting list of students trying to get into kindergarten through fourth grade classes. Work is still being done on the $25 million campus being built as part of the planned community of 10,800 homes known as River Islands at Lathrop.“We were surprised,” said Susan Dell’Osso, River Islands project manager and school board member. “There was standing room only at the orientations we held.”Altogether 550 have applied so far for classes that start in August.The academy already has all space in the kindergarten through fourth grades filled with 150 students on a waiting list.