Sorry, students and teachers: Apple Computer ( AAPL) no longer wants to sell you its discount desktop system. The company has ceased selling the $900 version of its iMac computer to individual customers just one week after announcing the new system. Apple spokeswoman Teresa Weaver declined to explain why the company decided to stop selling the cut-rate computer to individual students and teachers -- and declined even to acknowledge that the company had changed its policy. However, Apple clearly advertised the $900 iMac in its store for individual education customers last week when it announced the new machine. Now that the $900 computer is no longer available, students and teachers will have to spend at least $300 more to get an iMac, Apple's all-in-one consumer desktop machine. AppleInsider reported Apple's move earlier on Wednesday. Apple sales representatives told the trade publication that the company had originally wanted to limit sales of the discount iMac to institutions only, and that opening up sales to individual customers was a "mistake." In its announcement last week, Apple didn't specify whether the computer would be available to individuals or institutions, saying only that the company was offering the computer to "education customers." Indeed, a Web address in the announcement points to a page where customers can log in as either individual or institutional customers. The $900 computer replaced Apple's eMac, which was based on an earlier version of the iMac. Unlike its present policy with the discount iMac, Apple did offer the eMac to individual customers. The education market has long been a stronghold for Apple. The company garnered about 12% of its sales last year from education customers. Shares of Apple closed regular trading off $2.69, or 4.9%, to $52.96. In after-hours trading, the stock was off another 18 cents, or less than 1%, to $52.78.