I have developed a application for students that retrieves data (eating menus) from a official eating website of the whole University (13 restaurants located around the city). Actually I spend a month and a half of doing some reconnaissance and programming (getting information of that site) so that I could develop strong algorithms so that if they somehow change the html tags, position or anything else, my app will still work. There have been previous attempts of other people that had the idea (and programmers behind which had knowledge) of providing real time eating menus to students but they would't give them a contract to get raw data which would be easy to implement into an app with maybe not more then 50 lines of code. No one had the courage, to fetch the html data and said that this would be a never ending story and almost imposible and that the app would quickly fail.

The app (windows phone, android) works perfectly and I have put in on windows phone store but have hidden until next monday waiting to solve some bugs and release the official version (wp8+, android 2.3+)

My question is. Is this legal? and will their servers crash more frequently due to frequent html downloading from the app users? Could I get into trouble?

My app is completely free and I' m expecting 5000-10 000 users to dowload and use the app since there is a high interest for the app and since it is the first one in the city after many attempts.

The problem of users not using their site to have a look at the menu and to decide which restaurant has the best food that day is that it takes at least a minute to get to the actual menu and it is time consuming and some 80 percent don't know that there are available menus on their website. With my app after one click you get the menu instantly.

I'm assuming the app reaches out to the website directly. If you were concerned about the extra usage your app was going to inflect you could have your app call your website, where you cache the screen scraped data, refreshing the cache every x hours. Of course, that shifts the burden to your website. But if you're worried about blow back, that would help minimise it.
The increased usage of their website may not be as bad as you think. I'm assuming people interested in the app were already using the website, as you're just serving up that same data by another means.
The legality of it? tricky. As long as you're not hurting their bottom line or making public data that was private, you should be OK (thats not legal advice)

commented: tnx for the answer! +4