If you are not going to have a fixed place of business in Spain and your customers are most likely not Spanish and your are not going to be receiving fees/money from Spanish resident businesses then I might consider a UK Ltd company or Irish Ltd company (if you are Irish).
In both countries it is very easy to incorporate and there is very little in the way of tax filing and all compliance for both Companies House and tax can be DIY. Also there is high VAT exemption limits. Tax work can mostly be done on an annual basis and there is no hassle from either UK or Irish tax authorities and all can very easily be done on line.
Spain on the other hand is tedious, there is no VAT exemption limit, you need to file quarterly tax returns, you will most likely need to engage a gestoria or accountant, company incorporation is expensive, and you will have to pay high social insurance contributions.
Example
Set up Spanish SL – EUR1,500 to EUR2,000
Maintain Spanish SL – EUR2,000 to EUR3,000 pa
With the UK/Irish company route you collect fees how/where you want and you pass on to course provider less your booking fee/commission – you do it your way and no gesteria telling you you have to do it the Spanish way and telling you all your UK/Irish invoices are invalid etc ……consider where you will be incurring expenses etc
If the business takes off and starts generating sufficient income to pay you a salary then you could consider declaring any fees you take from the company and declaring in Spain which you should do if you live full-time in Spain and you remit that money to your Spanish bank account etc
But you do need to be careful if your presence in Spain is noticeable and if you have a fixed place of business where members of the public can visit etc because then you could be classed as a tour operator and as I said before this is a very regulated area.
You should consider your business plan and how much money you anticipate making in 1-2-3 years etc and whether you need to be paying Spanish social insurance etc.
Basically do your homework and know exactly the pros and cons of the various alternatives.