On the ISTEP+ Retest, is this just for grades 11 and 12?

Typically grades 11 and 12.  A 10th grade student that moves into the state after the fall testing can retest in the spring.

All the students in my school speak English, so in the Language Minority field should I mark them all as "Fluent English Proficient (FEP)" or "Native English Speaker"?

In most cases, neither.

Code 1 "Fluent English Proficient" is to be used only for language minority. Code 3 "Native English Speaker" is only for immigrant students. Most students in Indiana are not language minority or immigrant students and so in most cases, this field should be left blank.

I left the last field blank in my Excel file because I don't have any Language Minority students. When I save my file as CSV and submit it, I get an error about having an incorrect number of data elements. When I open my file in Notepad, there's nothing after Socio-economic Status (SES).

The simplest solution to this problem is to put an apostrophe (') - also known as a single quotation mark - in any cell in the last column (the "LM" column) that is intentionally left blank. (Tip: If the records with blank LM fields are all grouped together, you can put one apostrophe in the first row and copy it down.) This will tell Excel to fill in the comma to indicate a blank item at the end. (Note that you will not be able to see the apostrophe in the worksheet but if you go to the cell, you can see it in the formula editing bar.) STN Support can provide additional assistance if you intend to prepare your data file in Excel.