export-csv over writing 1st row

I have a powershell script that imports a list of students, ids, school, etc.. the script seems to work fine (i've stepped thru it in the debugger and i see it writing and overwriting the data) but instead of writing one line of data per line (row) it continues overwriting the first row. 

see code below:

Add-PSSnapin Quest.ActiveRoles.ADManagement

#import list of students from hourly IC extract

$users = Import-Csv C:\Users\edge.brandon\Desktop\enrollment\mbcextract.csv |where {$_.'grade' -ne 'PK' -and $_.'name' -ne 'Ombudsman' -and $_.'name' -ne 'Ombudsman MS' -and $_.'name' -ne 'z Transition Services' -and $_.'name' -ne 'z Home Services'}

# initialize an array that will be output

if ($users)

{

foreach ($u in $users) 

{

# sets middle name and initial to null so that variable does not cary over to next student if they have no middle name

$middle= $null

$mi= $null

$first= ($u.'firstname')

$last= ($u.'lastname')

$middle= ($u.'middlename')

$birth= ($u.'birthdate')

$grade= ($u.'grade')

$id= ($u.'studentNumber')

$schoolid= ($u.'sch.number') 

#Removes spaces, apostrophes, hyphens, periods, commas from name

$first=$first.Replace(" ", "")

$middle=$middle.Replace(" ", "")

$last=$last.Replace(" ", "")

$first=$first.Replace("'", "")

$middle=$middle.Replace("'", "")

$last=$last.Replace("'", "")

$first=$first.Replace("-", "")

$middle=$middle.Replace("-", "")

$last=$last.Replace("-", "")

$first=$first.Replace(".", "")

$middle=$middle.Replace(".", "")

$last=$last.Replace(".", "")

$first=$first.Replace(",", "")

$middle=$middle.Replace(",", "")

$last=$last.Replace(",", "")

# sets 1st and middle initial. also sets mmdd of birth

$fi= $first.substring(0,1)

$mi= $middle.substring(0,1)

$mmdd =$birth.substring(0,4)

#sets username and then makes sure it truncates anything after 20 characters

$un= ($last + $fi + $mi +$mmdd)

$un= $un.substring(0,20)

$u | select $id,$un,$first,$last,$schoolid,$grade,$upn,"1"," ","0" | export-csv mbc.csv -NoTypeInformation 

} 

} 

Remove-PSSnapin Quest.ActiveRoles.ADManagement