What is the standard practice if I can no longer find students that contributed to a paper that we would like to publish now? The students contributed data, but obviously had nothing to do with writing the paper. Still, I feel that they deserve authorship. I can no longer find a current email address for them. Should I put them only in the acknowledgements section?
This is a tricky one! Since some venues may require each author to sign over copyright, it complicates matters above and beyond the ethical question of what is right.
If you feel they deserve authorship, you should IMHO make a good faith effort to find them (web search, ask the university for contact info, etc). But in the end, if you can't find someone and they contributed data but not writing the paper, I see no way you can include them as an author. An acknowledgment would be appropriate.
The next answer assumes that the policy of the venue is silent about such situations (otherwise you simply follow the written guidelines). Moreover, let's assume that you failed to find the students or their representatives in reasonable time (otherwise contact these people and proceed from there). To find the people, you might consider asking the officials (administration, government, police, ...), though it might take more time and money than you originally expected. System administrators and secretaries may know more than they are allowed to say in public. Assuming all that fails:
If the students deserve the authorship, put them as authors, otherwise don't.
Of course, "deserve" is vague and open to interpretation, but this is a somewhat separate question. Naturally, most coauthors also co-write the paper. Still, a proof of a mathematical conjecture on a sheet of paper during lunchtime may be a substantial contribution deserving the (co)authorship, even if the coauthor does no paper writing. Similarly, the data your students contributed may be of great value or not. The value in the context counts, and there cannot be a general reply on that.
Next, you have to satisfy the formal requirements of a submission system. Inform the program chair (for conferences) or the main editor (for journals) of the situation and of your best efforts to contact the students. If you don't get an answer by the submission deadline but still have to enter the lost-coauthors' e-mail addresses into some form, use their old e-mails or, if that is not possible, reuse your own e-mail address or (at worst) create dummy e-mail boxes for the purpose of submission. State that you submit on the behalf of the coauthors and inform the chair/editor why you are doing so. (Once, I even had to temporarily add and then remove a new, dummy coauthor like Jane Doe to one of my papers because of certain technical limitations [which are unrelated here]. Upon informing the PC chair, it was not an issue.)
Note: regardless of whether you make the students the coauthors or not, the act of submission might involve risks. (See the comments.)
This is a difficult one, but I think in the end you have no choice but to put them in the acknowledgements if you cannot contact them, with an appropriate note regarding the level of their contributions. You should do your best to find them (social media? Contact other past students? Google?), but sometimes this might not be possible.
It is assumed that any author on a paper has given their consent to have their name attached to that particular piece of work. In your situation, the student(s) have not had this opportunity. I would be (rightly) upset if I found that someone had given me authorship on a paper without my consent. It would probably stop me doing any work with this person in future, and if I didn't like the published manuscript it would also involve a letter to the journal editor explaining the situation.
Even worse (I'm not suggesting this is the situation in your case), if the paper is of dubious quality, it may even hurt the student(s) in their future academic life to have their name attached to that paper.
Finally, I think most journals will have you sign off that each author has given their approval to the final manuscript. Are you happy lying about this? If this is discovered, prepare to face retraction of the paper.
I think it's a sign of a good supervisor that you value their contribution enough to want to give them authorship, even though you cannot contact them about it. It's just a shame that the rules of authorship preclude this.
Good luck with the paper!