Ever wonder who those high school students are practicing at CFA all weekend? It’s Boston Youth Symphony Orchestras (BYSO), now in its 53rd season, and tonight they are holding their inaugural spring Celebratory Event at Mandarin Oriental Boston from 6:30-9:00 PM. Along with their Honorary Host Committee Members Yo-Yo Ma, Boston Mayor Thomas M. Menino and Angela Menino and actor Stephen Lang, BYSO will host hundreds of guests to celebrate the achievements of BYSO’s young musicians. The centerpiece of the evening is a performance by fifty BYSO musicians led by Music Director, Federico Cortese. Tickets are $150 and can be purchased by calling 617-358-0280.