SI

 My high school (college preparatory), and a few more

St. Ignatius (in San Francisco), Silay Institute (a philippine college), supplementary instruction

smart intelligent, stupid ignorant, seriously ill, self-injury, swarm intelligence, synthetic intelligence

sports illustrated, skeptical inquirer, social improvement, smithsonian institute, systeme international (metric system), sport injected (honda), silicon, a chinese dessert, and socialist international

staten island, sleeping in, sexual intercourse, suck it, spanish for yes, homonymous with c, see, sea