I assume the visual analogue scale might be a good way to express stress levels, although it has probably never been applied in this context.. I therefore cannot guarantee you that it is the best possible scale for expressing learning stress among students. Among patients in hospitals it is a good way to ask about discomfort and pain. An example appears in the following link http://assessment-module.yale.edu/im-palliative/visual-analogue-scale
It would be possible to put the emoticons (i.e. the smileys) on one side and the numbers 0 to 10 on the other side. You can then ask the students to mark on the scale with the smileys how stressed they feel. Then you turn the scale and note the number corresponding to the mark, i.e. if you have a big smile, that would correspond to number 0, i.e. not stressed. Once you have collected the responses, you have a good idea on the average stress level at a given time. If you aim to analyse the data, you would need to apply an analysis corresponding to ordinal scale level, e.g. Kruskall-Wallis
If you aim, however, for an analysis which is based on a continuous scale level (e.g. analysis of variance), you could rather apply a track bar, which can be programmed. When using such a track bar, students would indicate how stressed they are at a given moment (e.g. from a number of 0 to a number of 100). The answers could then be saved for each student at a given time or over various time points (to see the development of stress levels, e.g. the stress levels may rise as they approach their exams). For an example on scales, refer to the following link: