I find a lot of people in Holland think 'a lot of' is too colloquial for use in academic work. Is that the case?
Let's put it this way: It's "dodgy" if you are doing a scientific paper and should be using more precise quantitative terminology. @kiamlaluno's suggestion of replacing it with "many" doesn't really improve precision in those cases, though.
I have seen "a lot of" used appropriately a lot of times in scholarly works, though, especially those dealing with the humanities rather than the sciences, and especially lately. Language changes, as we all know, and what were strict and uncompromising requirements 100 years ago are often relaxed today.
In fact, although I don't see why one couldn't use the expression as an aside or commentary on a piece of data, it does give a much less formal feel to a piece of writing, and may induce the reader not to take the statement seriously. Consider the second sentences in the following comparison:
The studies showed that 74.65% of the time rats injected with the compound rather than the placebo developed tumors. That's a lot of evidence in favor of considering the compound to be a carcinogen.
The studies showed that 74.65% of the time rats injected with the compound rather than the placebo developed tumors. That constitutes strong evidence in favor of considering the compound to be a carcinogen.
The tone of the second example is simply more in keeping with the dispassionate delivery of data, and is likely to be much better received by the academic community. So even if you are not stylistically prohibited from using "a lot of" it may make sense to avoid it in an academic context. And if you are going to school in Holland, and that is the prevailing sentiment you encounter, taking a rebellious stance on this issue may affect your progress negatively.