Searching on calculated fields via the web?

[AJB_UMASS]

OK - this is really puzzling me. I have a field that contains a calculated value - it has to do with student school records. It's a simple text calculation, and it works like so:

If (SchoolA not empty), equal to SchoolA,

else,

If (SchoolB not empty), equal to SchoolB,

else,

blank result.

When I try and have a pull-down index for searching this field via a web page, it obstinately refuses to do anything - the field is locked and allows no input at all, even by hand.

Any idea what I'm doing wrong?