String[] chkId = request.getParameterValues("projectId");
String sql = "delete from t_student where checkbox=?";
String sql2 = " or checkbox=?";

Connection conn = ....;
PreparedStatement pstmt = ....;
pstmt = conn.prepareStatement(sql);
for(int i=0;i<chkId.length;i++){
   //假想的实现功能      
   sql = sql + sql2*（chkId.length-1）;
}

delete from t_student where checkbox =? or checkbox=? or checkbox=?......

String sql = "delete from t_student where 1=1";
String condition = "AND checkbok in (";
for(int i=0;i<chkId.length;i++){
    condition += "?,"
}
condition = condition.subString(0, condition.length() - 1);
condition += ")"