Is Excel sufficient for data science?
First of all check out this post. It has many reasons why Excel is inferior to other solutions, regarding data science tasks. Excel also can't handle large datasets (hundreds of thousands of records - not to mention anything in the vicinity of Big Data), image and sound data.

Excel is good for simple tasks concerning spreadsheets; it emphasizes more on presentation and ease of use, while having minimal support for actually analysing the data. Unless all you want to do is to calculate simple statistical measures (mean, average, etc) or building a very simple model (e.g. linear regression), Excel is inefficient. That being said, 99% the work a company has to deal with concerning data is simple enough to be manageable through Excel.

However Data Science mainly deals with regression, classification and complex models that excel isn't equipped to handle! If your students want to have a look at data science you need to teach them a tool that will be useful to them (R, python, etc.). These languages also have libraries with tons of built in models to "play with".

Another really huge reason I would go with the latter options is that they are open source. I personally feel that open source software should be preferred from an educational standpoint to proprietary solutions (this is also why I suggest python and R over Matlab)!