and case when @st is not null then STUDENTS.STUDENTS_ID in @st

declare @DBK_SYS_VAR_df datetime

declare @DBK_SYS_VAR_dt datetime

declare @d datetime
set @d = GETDATE()
declare @DBK_SYS_VAR_F3 money
set @DBK_SYS_VAR_F3 = 0
declare @id int
declare @DATE_OSTATOK datetime
set @DATE_OSTATOK = {ts '2017-01-01 00:00:00.000'}

declare @st  int
set @st = 100

set @DBK_SYS_VAR_df = '2018-01-01 00:00:00.000'
set @DBK_SYS_VAR_dt = '2018-01-11 00:00:00.000'

SELECT  
 STUDENTS.STUDENTS_ID, FM_CONTR.FM_CONTR_ID, 
  isnull((SELECT sum(at.TRAN_SUM)
FROM FM_ACCOUNT_TRAN at WITH(NOLOCK)
join FM_ACCOUNT_TRAN at2 WITH(NOLOCK) on at.FM_MAIN_TRAN_ID = at2.fm_account_tran_id 
where at2.FM_CONTR_ID = FM_CONTR.FM_CONTR_ID 
  AND (at.TRAN_DATE >= isnull(@DATE_OSTATOK, '2000-01-01 00:00:00.000') and at.TRAN_DATE < @DBK_SYS_VAR_df ) 
  and at.TRAN_TYPE in ('J','K','Z', 'I', 'A','W','O','B')), 0) ostatok_nop, @DBK_SYS_VAR_df df, @DBK_SYS_VAR_dt dt
FROM
 STUDENTS STUDENTS WITH(NOLOCK)  
 JOIN FM_CLINK_STUDENTS FM_CLINK_STUDENTS WITH(NOLOCK)  ON STUDENTS.STUDENTS_ID = FM_CLINK_STUDENTS.STUDENTS_ID 
 JOIN FM_CLINK FM_CLINK WITH(NOLOCK)  ON FM_CLINK.FM_CLINK_ID = FM_CLINK_STUDENTS.FM_CLINK_ID 
 JOIN FM_CONTR FM_CONTR WITH(NOLOCK)  ON FM_CONTR.FM_CONTR_ID = FM_CLINK.FM_CONTR_ID 
WHERE
 (FM_CONTR.DEPOSIT=1) and ........

STUDENTS.STUDENTS_ID in ( @st) or (coalesce (@st, -1) = -1)