Fifteen more companies, including Google and the YouTube-based educational organization Khan Academy,  have signed on to a pledge to protect student privacy. The pledge was highlighted in a speech by President Obama last week, in which he also said he will introduce legislation to protect data collected in the classroom.

The two companies, both major players in education technology, are among second wave of 15 that signed on to the pledge Monday; 75 signed the agreement last week.  The document  holds companies to several data privacy tenets, including promises not to sell student information or to use behaviorally targeted advertising on education products. It also promises to make it easy for parents to see their students' data and to be transparent about how those data are collected and used.

Major education technology firms including Apple, Microsoft and  textbook publisher Houghton Mifflin Harcourt were among those that signed on ahead of the president's speech. The move was hailed by privacy advocates, but  many commented on the fact that Google and Amazon were conspicuously absent from the list.

Obama encouraged all firms to join the pledge --   a project of the Future of Privacy Forum and The Software & Information Industry Association -- and said the government will "make sure" parents knew which companies had not signed. In the same speech, the president said he would put forth a legislative proposal to ensure information collected through the classroom is only used for educational purposes.

Google already had some of those policies in place after facing criticism last year for providing advertisements on its "Apps for Education" products. In a blog post last April, the company said it would remove the option to enable ads on those services altogether. Google also said that it would no longer be able to collect or use student data for advertising purposes.

The announcement, made on the Future of Privacy Forum Web site, did not elaborate on why the companies waited to sign the pledge until after last week's speech.

Google, Khan and Amazon did not immediately respond to requests for comment.